1. 4.6 Links
      1. 4.6.1 Introduction
      2. 4.6.2 Links created by a and area elements
      3. 4.6.3 API for a and area elements
      4. 4.6.4 Following hyperlinks
      5. 4.6.5 Downloading resources
      6. 4.6.6 Hyperlink auditing
        1. 4.6.6.1 The `Ping-From` and `Ping-To` headers
      7. 4.6.7 Link types
        1. 4.6.7.1 Link type "alternate"
        2. 4.6.7.2 Link type "author"
        3. 4.6.7.3 Link type "bookmark"
        4. 4.6.7.4 Link type "canonical"
        5. 4.6.7.5 Link type "dns-prefetch"
        6. 4.6.7.6 Link type "expect"
        7. 4.6.7.7 Link type "external"
        8. 4.6.7.8 Link type "help"
        9. 4.6.7.9 Link type "icon"
        10. 4.6.7.10 Link type "license"
        11. 4.6.7.11 Link type "manifest"
        12. 4.6.7.12 Link type "modulepreload"
        13. 4.6.7.13 Link type "nofollow"
        14. 4.6.7.14 Link type "noopener"
        15. 4.6.7.15 Link type "noreferrer"
        16. 4.6.7.16 Link type "opener"
        17. 4.6.7.17 Link type "pingback"
        18. 4.6.7.18 Link type "preconnect"
        19. 4.6.7.19 Link type "prefetch"
        20. 4.6.7.20 Link type "preload"
        21. 4.6.7.21 Link type "privacy-policy"
        22. 4.6.7.22 Link type "search"
        23. 4.6.7.23 Link type "stylesheet"
        24. 4.6.7.24 Link type "tag"
        25. 4.6.7.25 Link Type "terms-of-service"
        26. 4.6.7.26 Sequential link types
          1. 4.6.7.26.1 Link type "next"
          2. 4.6.7.26.2 Link type "prev"
        27. 4.6.7.27 Other link types

4.6.1 Introduction

Links are a conceptual construct, created by a, area, form, and link elements, that represent a connection between two resources, one of which is the current Document. There are three kinds of links in HTML:

Links to external resources

These are links to resources that are to be used to augment the current document, generally automatically processed by the user agent. All external resource links have a fetch and process the linked resource algorithm which describes how the resource is obtained.

Hyperlinks

These are links to other resources that are generally exposed to the user by the user agent so that the user can cause the user agent to navigate to those resources, e.g. to visit them in a browser or download them.

Internal resource links

These are links to resources within the current document, used to give those resources special meaning or behavior.

For link elements with an href attribute and a rel attribute, links must be created for the keywords of the rel attribute, as defined for those keywords in the link types section.

Similarly, for a and area elements with an href attribute and a rel attribute, links must be created for the keywords of the rel attribute as defined for those keywords in the link types section. Unlike link elements, however, a and area elements with an href attribute that either do not have a rel attribute, or whose rel attribute has no keywords that are defined as specifying hyperlinks, must also create a hyperlink. This implied hyperlink has no special meaning (it has no link type) beyond linking the element's node document to the resource given by the element's href attribute.

Similarly, for form elements with a rel attribute, links must be created for the keywords of the rel attribute as defined for those keywords in the link types section. form elements that do not have a rel attribute, or whose rel attribute has no keywords that are defined as specifying hyperlinks, must also create a hyperlink.

A hyperlink can have one or more hyperlink annotations that modify the processing semantics of that hyperlink.

The href attribute on a and area elements must have a value that is a valid URL potentially surrounded by spaces.

The href attribute on a and area elements is not required; when those elements do not have href attributes they do not create hyperlinks.

The target attribute, if present, must be a valid navigable target name or keyword. It gives the name of the navigable that will be used. User agents use this name when following hyperlinks.

The download attribute, if present, indicates that the author intends the hyperlink to be used for downloading a resource. The attribute may have a value; the value, if any, specifies the default filename that the author recommends for use in labeling the resource in a local file system. There are no restrictions on allowed values, but authors are cautioned that most file systems have limitations with regard to what punctuation is supported in filenames, and user agents are likely to adjust filenames accordingly.

Element/a#attr-ping

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The ping attribute, if present, gives the URLs of the resources that are interested in being notified if the user follows the hyperlink. The value must be a set of space-separated tokens, each of which must be a valid non-empty URL whose scheme is an HTTP(S) scheme. The value is used by the user agent for hyperlink auditing.

The rel attribute on a and area elements controls what kinds of links the elements create. The attribute's value must be an unordered set of unique space-separated tokens. The allowed keywords and their meanings are defined below.

rel's supported tokens are the keywords defined in HTML link types which are allowed on a and area elements, impact the processing model, and are supported by the user agent. The possible supported tokens are noreferrer, noopener, and opener. rel's supported tokens must only include the tokens from this list that the user agent implements the processing model for.

The rel attribute has no default value. If the attribute is omitted or if none of the values in the attribute are recognized by the user agent, then the document has no particular relationship with the destination resource other than there being a hyperlink between the two.

The hreflang attribute on a elements that create hyperlinks, if present, gives the language of the linked resource. It is purely advisory. The value must be a valid BCP 47 language tag. [BCP47] User agents must not consider this attribute authoritative — upon fetching the resource, user agents must use only language information associated with the resource to determine its language, not metadata included in the link to the resource.

The type attribute, if present, gives the MIME type of the linked resource. It is purely advisory. The value must be a valid MIME type string. User agents must not consider the type attribute authoritative — upon fetching the resource, user agents must not use metadata included in the link to the resource to determine its type.

The referrerpolicy attribute is a referrer policy attribute. Its purpose is to set the referrer policy used when following hyperlinks. [REFERRERPOLICY]


When an a or area element's activation behavior is invoked, the user agent may allow the user to indicate a preference regarding whether the hyperlink is to be used for navigation or whether the resource it specifies is to be downloaded.

In the absence of a user preference, the default should be navigation if the element has no download attribute, and should be to download the specified resource if it does.

The activation behavior of an a or area element element given an event event is:

  1. If element has no href attribute, then return.

  2. Let hyperlinkSuffix be null.

  3. If element is an a element, and event's target is an img with an ismap attribute specified, then:

    1. Let x and y be 0.

    2. If event's isTrusted attribute is initialized to true, then set x to the distance in CSS pixels from the left edge of the image to the location of the click, and set y to the distance in CSS pixels from the top edge of the image to the location of the click.

    3. If x is negative, set x to 0.

    4. If y is negative, set y to 0.

    5. Set hyperlinkSuffix to the concatenation of U+003F (?), the value of x expressed as a base-ten integer using ASCII digits, U+002C (,), and the value of y expressed as a base-ten integer using ASCII digits.

  4. Let userInvolvement be event's user navigation involvement.

  5. If the user has expressed a preference to download the hyperlink, then set userInvolvement to "browser UI".

    That is, if the user has expressed a specific preference for downloading, this no longer counts as merely "activation".

  6. If element has a download attribute, or if the user has expressed a preference to download the hyperlink, then download the hyperlink created by element with hyperlinkSuffix set to hyperlinkSuffix and userInvolvement set to userInvolvement.

  7. Otherwise, follow the hyperlink created by element with hyperlinkSuffix set to hyperlinkSuffix and userInvolvement set to userInvolvement.

4.6.3 API for a and area elements

interface mixin HTMLHyperlinkElementUtils {
  [CEReactions] stringifier attribute USVString href;
  readonly attribute USVString origin;
  [CEReactions] attribute USVString protocol;
  [CEReactions] attribute USVString username;
  [CEReactions] attribute USVString password;
  [CEReactions] attribute USVString host;
  [CEReactions] attribute USVString hostname;
  [CEReactions] attribute USVString port;
  [CEReactions] attribute USVString pathname;
  [CEReactions] attribute USVString search;
  [CEReactions] attribute USVString hash;
};
hyperlink.toString()
hyperlink.href

HTMLAnchorElement/href

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HTMLAnchorElement/toString

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HTMLAreaElement/href

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HTMLAreaElement/toString

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Returns the hyperlink's URL.

Can be set, to change the URL.

hyperlink.origin

HTMLAnchorElement/origin

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HTMLAreaElement/origin

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Returns the hyperlink's URL's origin.

hyperlink.protocol

HTMLAnchorElement/protocol

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HTMLAreaElement/protocol

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Returns the hyperlink's URL's scheme.

Can be set, to change the URL's scheme.

hyperlink.username

HTMLAnchorElement/username

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HTMLAreaElement/username

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Returns the hyperlink's URL's username.

Can be set, to change the URL's username.

hyperlink.password

HTMLAnchorElement/password

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HTMLAreaElement/password

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Returns the hyperlink's URL's password.

Can be set, to change the URL's password.

hyperlink.host

HTMLAnchorElement/host

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HTMLAreaElement/host

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Returns the hyperlink's URL's host and port (if different from the default port for the scheme).

Can be set, to change the URL's host and port.

hyperlink.hostname

HTMLAnchorElement/hostname

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HTMLAreaElement/hostname

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Returns the hyperlink's URL's host.

Can be set, to change the URL's host.

hyperlink.port

HTMLAnchorElement/port

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HTMLAreaElement/port

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Returns the hyperlink's URL's port.

Can be set, to change the URL's port.

hyperlink.pathname

HTMLAnchorElement/pathname

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HTMLAreaElement/pathname

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Returns the hyperlink's URL's path.

Can be set, to change the URL's path.

hyperlink.search

HTMLAnchorElement/search

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HTMLAreaElement/search

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Returns the hyperlink's URL's query (includes leading "?" if non-empty).

Can be set, to change the URL's query (ignores leading "?").

hyperlink.hash

HTMLAnchorElement/hash

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HTMLAreaElement/hash

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Returns the hyperlink's URL's fragment (includes leading "#" if non-empty).

Can be set, to change the URL's fragment (ignores leading "#").

An element implementing the HTMLHyperlinkElementUtils mixin has an associated url (null or a URL). It is initially null.

An element implementing the HTMLHyperlinkElementUtils mixin has an associated set the url algorithm, which runs these steps:

  1. Set this element's url to null.

  2. If this element's href content attribute is absent, then return.

  3. Let url be the result of encoding-parsing a URL given this element's href content attribute's value, relative to this element's node document.

  4. If url is not failure, then set this element's url to url.

When elements implementing the HTMLHyperlinkElementUtils mixin are created, and whenever those elements have their href content attribute set, changed, or removed, the user agent must set the url.

This is only observable for blob: URLs as parsing them involves a Blob URL Store lookup.

An element implementing the HTMLHyperlinkElementUtils mixin has an associated reinitialize url algorithm, which runs these steps:

  1. If element's url is non-null, its scheme is "blob", and it has an opaque path, then terminate these steps.

  2. Set the url.

To update href, set the element's href content attribute's value to the element's url, serialized.


The href getter steps are:

  1. Reinitialize url.

  2. Let url be this's url.

  3. If url is null and this has no href content attribute, return the empty string.

  4. Otherwise, if url is null, return this's href content attribute's value.

  5. Return url, serialized.

The href setter steps are to set this's href content attribute's value to the given value.

The origin getter steps are:

  1. Reinitialize url.

  2. If this's url is null, return the empty string.

  3. Return the serialization of this's url's origin.

The protocol getter steps are:

  1. Reinitialize url.

  2. If this's url is null, return ":".

  3. Return this's url's scheme, followed by ":".

The protocol setter steps are:

  1. Reinitialize url.

  2. If this's url is null, then return.

  3. Basic URL parse the given value, followed by ":", with this's url as url and scheme start state as state override.

    Because the URL parser ignores multiple consecutive colons, providing a value of "https:" (or even "https::::") is the same as providing a value of "https".

  4. Update href.

The username getter steps are:

  1. Reinitialize url.

  2. If this's url is null, return the empty string.

  3. Return this's url's username.

The username setter steps are:

  1. Reinitialize url.

  2. Let url be this's url.

  3. If url is null or url cannot have a username/password/port, then return.

  4. Set the username, given url and the given value.

  5. Update href.

The password getter steps are:

  1. Reinitialize url.

  2. Let url be this's url.

  3. If url is null, then return the empty string.

  4. Return url's password.

The password setter steps are:

  1. Reinitialize url.

  2. Let url be this's url.

  3. If url is null or url cannot have a username/password/port, then return.

  4. Set the password, given url and the given value.

  5. Update href.

The host getter steps are:

  1. Reinitialize url.

  2. Let url be this's url.

  3. If url or url's host is null, return the empty string.

  4. If url's port is null, return url's host, serialized.

  5. Return url's host, serialized, followed by ":" and url's port, serialized.

The host setter steps are:

  1. Reinitialize url.

  2. Let url be this's url.

  3. If url is null or url has an opaque path, then return.

  4. Basic URL parse the given value, with url as url and host state as state override.

  5. Update href.

The hostname getter steps are:

  1. Reinitialize url.

  2. Let url be this's url.

  3. If url or url's host is null, return the empty string.

  4. Return url's host, serialized.

The hostname setter steps are:

  1. Reinitialize url.

  2. Let url be this's url.

  3. If url is null or url has an opaque path, then return.

  4. Basic URL parse the given value, with url as url and hostname state as state override.

  5. Update href.

The port getter steps are:

  1. Reinitialize url.

  2. Let url be this's url.

  3. If url or url's port is null, return the empty string.

  4. Return url's port, serialized.

The port setter steps are:

  1. Reinitialize url.

  2. Let url be this's url.

  3. If url is null or url cannot have a username/password/port, then return.

  4. If the given value is the empty string, then set url's port to null.

  5. Otherwise, basic URL parse the given value, with url as url and port state as state override.

  6. Update href.

The pathname getter steps are:

  1. Reinitialize url.

  2. Let url be this's url.

  3. If url is null, then return the empty string.

  4. Return the result of URL path serializing url.

The pathname setter steps are:

  1. Reinitialize url.

  2. Let url be this's url.

  3. If url is null or url has an opaque path, then return.

  4. Set url's path to the empty list.

  5. Basic URL parse the given value, with url as url and path start state as state override.

  6. Update href.

The search getter steps are:

  1. Reinitialize url.

  2. Let url be this's url.

  3. If url is null, or url's query is either null or the empty string, return the empty string.

  4. Return "?", followed by url's query.

The search setter steps are:

  1. Reinitialize url.

  2. Let url be this's url.

  3. If url is null, terminate these steps.

  4. If the given value is the empty string, set url's query to null.

  5. Otherwise:

    1. Let input be the given value with a single leading "?" removed, if any.

    2. Set url's query to the empty string.

    3. Basic URL parse input, with url as url and query state as state override.

  6. Update href.

The hash getter steps are:

  1. Reinitialize url.

  2. Let url be this's url.

  3. If url is null, or url's fragment is either null or the empty string, return the empty string.

  4. Return "#", followed by url's fragment.

The hash setter steps are:

  1. Reinitialize url.

  2. Let url be this's url.

  3. If url is null, then return.

  4. If the given value is the empty string, set url's fragment to null.

  5. Otherwise:

    1. Let input be the given value with a single leading "#" removed, if any.

    2. Set url's fragment to the empty string.

    3. Basic URL parse input, with url as url and fragment state as state override.

  6. Update href.

An element element cannot navigate if any of the following are true:

This is also used by form submission for the form element. The exception for a elements is for compatibility with web content.

To get an element's noopener, given an a, area, or form element element and a string target:

  1. If element's link types include the noopener or noreferrer keyword, then return true.

  2. If element's link types do not include the opener keyword and target is an ASCII case-insensitive match for "_blank", then return true.

  3. Return false.

To follow the hyperlink created by an element subject, given an optional hyperlinkSuffix (default null) and an optional userInvolvement (default "none"):

  1. If subject cannot navigate, then return.

  2. Let replace be false.

  3. Let targetAttributeValue be the empty string.

  4. If subject is an a or area element, then set targetAttributeValue to the result of getting an element's target given subject.

  5. Let noopener be the result of getting an element's noopener with subject and targetAttributeValue.

  6. Let targetNavigable be the first return value of applying the rules for choosing a navigable given targetAttributeValue, subject's node navigable, and noopener.

  7. If targetNavigable is null, then return.

  8. Let urlString be the result of encoding-parsing-and-serializing a URL given subject's href attribute value, relative to subject's node document.

  9. If urlString is failure, then return.

  10. If hyperlinkSuffix is non-null, then append it to urlString.

  11. Let referrerPolicy be the current state of subject's referrerpolicy content attribute.

  12. If subject's link types includes the noreferrer keyword, then set referrerPolicy to "no-referrer".

  13. Navigate targetNavigable to urlString using subject's node document, with referrerPolicy set to referrerPolicy and userInvolvement set to userInvolvement.

    Unlike many other types of navigations, following hyperlinks does not have special "replace" behavior for when documents are not completely loaded. This is true for both user-initiated instances of following hyperlinks, as well as script-triggered ones via, e.g., aElement.click().

4.6.5 Downloading resources

HTMLAnchorElement/download

Support in all current engines.

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Opera?Edge79+
Edge (Legacy)13+Internet ExplorerNo
Firefox Android?Safari iOS?Chrome Android?WebView Android?Samsung Internet?Opera Android?

In some cases, resources are intended for later use rather than immediate viewing. To indicate that a resource is intended to be downloaded for use later, rather than immediately used, the download attribute can be specified on the a or area element that creates the hyperlink to that resource.

The attribute can furthermore be given a value, to specify the filename that user agents are to use when storing the resource in a file system. This value can be overridden by the `Content-Disposition` HTTP header's filename parameters. [RFC6266]

In cross-origin situations, the download attribute has to be combined with the `Content-Disposition` HTTP header, specifically with the attachment disposition type, to avoid the user being warned of possibly nefarious activity. (This is to protect users from being made to download sensitive personal or confidential information without their full understanding.)


To download the hyperlink created by an element subject, given an optional hyperlinkSuffix (default null) and an optional userInvolvement (default "none"):

  1. If subject cannot navigate, then return.

  2. If subject's node document's active sandboxing flag set has the sandboxed downloads browsing context flag set, then return.

  3. Let urlString be the result of encoding-parsing-and-serializing a URL given subject's href attribute value, relative to subject's node document.

  4. If urlString is failure, then return.

  5. If hyperlinkSuffix is non-null, then append it to urlString.

  6. If userInvolvement is not "browser UI", then:

    1. Assert: subject has a download attribute.

    2. Let navigation be subject's relevant global object's navigation API.

    3. Let filename be the value of subject's download attribute.

    4. Let continue be the result of firing a download request navigate event at navigation with destinationURL set to urlString, userInvolvement set to userInvolvement, and filename set to filename.

    5. If continue is false, then return.

  7. Run these steps in parallel:

    1. Optionally, the user agent may abort these steps, if it believes doing so would safeguard the user from a potentially hostile download.

    2. Let request be a new request whose URL is urlString, client is entry settings object, initiator is "download", destination is the empty string, and whose synchronous flag and use-URL-credentials flag are set.

    3. Handle the result of fetching request as a download.

When a user agent is to handle a resource obtained from a fetch as a download, it should provide the user with a way to save the resource for later use, if a resource is successfully obtained. Otherwise, it should report any problems downloading the file to the user.

If the user agent needs a filename for a resource being handled as a download, it should select one using the following algorithm.

This algorithm is intended to mitigate security dangers involved in downloading files from untrusted sites, and user agents are strongly urged to follow it.

  1. Let filename be the undefined value.

  2. If the resource has a `Content-Disposition` header, that header specifies the attachment disposition type, and the header includes filename information, then let filename have the value specified by the header, and jump to the step labeled sanitize below. [RFC6266]

  3. Let interface origin be the origin of the Document in which the download or navigate action resulting in the download was initiated, if any.

  4. Let resource origin be the origin of the URL of the resource being downloaded, unless that URL's scheme component is data, in which case let resource origin be the same as the interface origin, if any.

  5. If there is no interface origin, then let trusted operation be true. Otherwise, let trusted operation be true if resource origin is the same origin as interface origin, and false otherwise.

  6. If trusted operation is true and the resource has a `Content-Disposition` header and that header includes filename information, then let filename have the value specified by the header, and jump to the step labeled sanitize below. [RFC6266]

  7. If the download was not initiated from a hyperlink created by an a or area element, or if the element of the hyperlink from which it was initiated did not have a download attribute when the download was initiated, or if there was such an attribute but its value when the download was initiated was the empty string, then jump to the step labeled no proposed filename.

  8. Let proposed filename have the value of the download attribute of the element of the hyperlink that initiated the download at the time the download was initiated.

  9. If trusted operation is true, let filename have the value of proposed filename, and jump to the step labeled sanitize below.

  10. If the resource has a `Content-Disposition` header and that header specifies the attachment disposition type, let filename have the value of proposed filename, and jump to the step labeled sanitize below. [RFC6266]

  11. No proposed filename: If trusted operation is true, or if the user indicated a preference for having the resource in question downloaded, let filename have a value derived from the URL of the resource in an implementation-defined manner, and jump to the step labeled sanitize below.

  12. Let filename be set to the user's preferred filename or to a filename selected by the user agent, and jump to the step labeled sanitize below.

    If the algorithm reaches this step, then a download was begun from a different origin than the resource being downloaded, and the origin did not mark the file as suitable for downloading, and the download was not initiated by the user. This could be because a download attribute was used to trigger the download, or because the resource in question is not of a type that the user agent supports.

    This could be dangerous, because, for instance, a hostile server could be trying to get a user to unknowingly download private information and then re-upload it to the hostile server, by tricking the user into thinking the data is from the hostile server.

    Thus, it is in the user's interests that the user be somehow notified that the resource in question comes from quite a different source, and to prevent confusion, any suggested filename from the potentially hostile interface origin should be ignored.

  13. Sanitize: Optionally, allow the user to influence filename. For example, a user agent could prompt the user for a filename, potentially providing the value of filename as determined above as a default value.

  14. Adjust filename to be suitable for the local file system.

    For example, this could involve removing characters that are not legal in filenames, or trimming leading and trailing whitespace.

  15. If the platform conventions do not in any way use extensions to determine the types of file on the file system, then return filename as the filename.

  16. Let claimed type be the type given by the resource's Content-Type metadata, if any is known. Let named type be the type given by filename's extension, if any is known. For the purposes of this step, a type is a mapping of a MIME type to an extension.

  17. If named type is consistent with the user's preferences (e.g., because the value of filename was determined by prompting the user), then return filename as the filename.

  18. If claimed type and named type are the same type (i.e., the type given by the resource's Content-Type metadata is consistent with the type given by filename's extension), then return filename as the filename.

  19. If the claimed type is known, then alter filename to add an extension corresponding to claimed type.

    Otherwise, if named type is known to be potentially dangerous (e.g. it will be treated by the platform conventions as a native executable, shell script, HTML application, or executable-macro-capable document) then optionally alter filename to add a known-safe extension (e.g. ".txt").

    This last step would make it impossible to download executables, which might not be desirable. As always, implementers are forced to balance security and usability in this matter.

  20. Return filename as the filename.

For the purposes of this algorithm, a file extension consists of any part of the filename that platform conventions dictate will be used for identifying the type of the file. For example, many operating systems use the part of the filename following the last dot (".") in the filename to determine the type of the file, and from that the manner in which the file is to be opened or executed.

User agents should ignore any directory or path information provided by the resource itself, its URL, and any download attribute, in deciding where to store the resulting file in the user's file system.

If a hyperlink created by an a or area element has a ping attribute, and the user follows the hyperlink, and the value of the element's href attribute can be parsed, relative to the element's node document, without failure, then the user agent must take the ping attribute's value, split that string on ASCII whitespace, parse each resulting token, relative to the element's node document, and then run these steps for each resulting URL ping URL, ignoring when parsing returns failure:

  1. If ping URL's scheme is not an HTTP(S) scheme, then return.

  2. Optionally, return. (For example, the user agent might wish to ignore any or all ping URLs in accordance with the user's expressed preferences.)

  3. Let settingsObject be the element's node document's relevant settings object.

  4. Let request be a new request whose URL is ping URL, method is `POST`, header list is « (`Content-Type`, `text/ping`) », body is `PING`, client is settingsObject, destination is the empty string, credentials mode is "include", referrer is "no-referrer", and whose use-URL-credentials flag is set, and whose initiator type is "ping".

  5. Let target URL be the result of encoding-parsing-and-serializing a URL given the element's href attribute's value, relative to the element's node document, and then:

    If the URL of the Document object containing the hyperlink being audited and ping URL have the same origin
    If the origins are different, but the scheme of the URL of the Document containing the hyperlink being audited is not "https"
    request must include a `Ping-From` header with, as its value, the URL of the document containing the hyperlink, and a `Ping-To` HTTP header with, as its value, the target URL.
    Otherwise
    request must include a `Ping-To` HTTP header with, as its value, target URL. request does not include a `Ping-From` header.
  6. Fetch request.

This may be done in parallel with the primary fetch, and is independent of the result of that fetch.

User agents should allow the user to adjust this behavior, for example in conjunction with a setting that disables the sending of HTTP `Referer` (sic) headers. Based on the user's preferences, UAs may either ignore the ping attribute altogether, or selectively ignore URLs in the list (e.g. ignoring any third-party URLs); this is explicitly accounted for in the steps above.

User agents must ignore any entity bodies returned in the responses. User agents may close the connection prematurely once they start receiving a response body.

When the ping attribute is present, user agents should clearly indicate to the user that following the hyperlink will also cause secondary requests to be sent in the background, possibly including listing the actual target URLs.

For example, a visual user agent could include the hostnames of the target ping URLs along with the hyperlink's actual URL in a status bar or tooltip.

The ping attribute is redundant with pre-existing technologies like HTTP redirects and JavaScript in allowing web pages to track which off-site links are most popular or allowing advertisers to track click-through rates.

However, the ping attribute provides these advantages to the user over those alternatives:

Thus, while it is possible to track users without this feature, authors are encouraged to use the ping attribute so that the user agent can make the user experience more transparent.

4.6.6.1 The `Ping-From` and `Ping-To` headers

The `Ping-From` and `Ping-To` HTTP request headers are included in hyperlink auditing requests. Their value is a URL, serialized.

4.6.7 Link types

Link_types

Support in all current engines.

Firefox1+Safari1+Chrome1+
Opera9+Edge79+
Edge (Legacy)12+Internet ExplorerYes
Firefox Android?Safari iOS?Chrome Android?WebView Android?Samsung Internet?Opera Android10.1+

Link_types

The following table summarizes the link types that are defined by this specification, by their corresponding keywords. This table is non-normative; the actual definitions for the link types are given in the next few sections.

In this section, the term referenced document refers to the resource identified by the element representing the link, and the term current document refers to the resource within which the element representing the link finds itself.

To determine which link types apply to a link, a, area, or form element, the element's rel attribute must be split on ASCII whitespace. The resulting tokens are the keywords for the link types that apply to that element.

Except where otherwise specified, a keyword must not be specified more than once per rel attribute.

Some of the sections that follow the table below list synonyms for certain keywords. The indicated synonyms are to be handled as specified by user agents, but must not be used in documents (for example, the keyword "copyright").

Keywords are always ASCII case-insensitive, and must be compared as such.

Thus, rel="next" is the same as rel="NEXT".

Keywords that are body-ok affect whether link elements are allowed in the body. The body-ok keywords are dns-prefetch, modulepreload, pingback, preconnect, prefetch, preload, and stylesheet.

New link types that are to be implemented by web browsers are to be added to this standard. The remainder can be registered as extensions.

4.6.7.1 Link type "alternate"

Alternative_style_sheets

Support in one engine only.

Firefox3+Safari?Chrome1–48
OperaYesEdgeNo
Edge (Legacy)?Internet Explorer8+
Firefox Android?Safari iOS?Chrome Android?WebView Android?Samsung Internet?Opera Android?

The alternate keyword may be used with link, a, and area elements.

The meaning of this keyword depends on the values of the other attributes.

If the element is a link element and the rel attribute also contains the keyword stylesheet

The alternate keyword modifies the meaning of the stylesheet keyword in the way described for that keyword. The alternate keyword does not create a link of its own.

Here, a set of link elements provide some style sheets:

<!-- a persistent style sheet -->
<link rel="stylesheet" href="default.css">

<!-- the preferred alternate style sheet -->
<link rel="stylesheet" href="green.css" title="Green styles">

<!-- some alternate style sheets -->
<link rel="alternate stylesheet" href="contrast.css" title="High contrast">
<link rel="alternate stylesheet" href="big.css" title="Big fonts">
<link rel="alternate stylesheet" href="wide.css" title="Wide screen">
If the alternate keyword is used with the type attribute set to the value application/rss+xml or the value application/atom+xml

The keyword creates a hyperlink referencing a syndication feed (though not necessarily syndicating exactly the same content as the current page).

For the purposes of feed autodiscovery, user agents should consider all link elements in the document with the alternate keyword used and with their type attribute set to the value application/rss+xml or the value application/atom+xml. If the user agent has the concept of a default syndication feed, the first such element (in tree order) should be used as the default.

The following link elements give syndication feeds for a blog:

<link rel="alternate" type="application/atom+xml" href="posts.xml" title="Cool Stuff Blog">
<link rel="alternate" type="application/atom+xml" href="posts.xml?category=robots" title="Cool Stuff Blog: robots category">
<link rel="alternate" type="application/atom+xml" href="comments.xml" title="Cool Stuff Blog: Comments">

Such link elements would be used by user agents engaged in feed autodiscovery, with the first being the default (where applicable).

The following example offers various different syndication feeds to the user, using a elements:

<p>You can access the planets database using Atom feeds:</p>
<ul>
 <li><a href="recently-visited-planets.xml" rel="alternate" type="application/atom+xml">Recently Visited Planets</a></li>
 <li><a href="known-bad-planets.xml" rel="alternate" type="application/atom+xml">Known Bad Planets</a></li>
 <li><a href="unexplored-planets.xml" rel="alternate" type="application/atom+xml">Unexplored Planets</a></li>
</ul>

These links would not be used in feed autodiscovery.

Otherwise

The keyword creates a hyperlink referencing an alternate representation of the current document.

The nature of the referenced document is given by the hreflang, and type attributes.

If the alternate keyword is used with the hreflang attribute, and that attribute's value differs from the document element's language, it indicates that the referenced document is a translation.

If the alternate keyword is used with the type attribute, it indicates that the referenced document is a reformulation of the current document in the specified format.

The hreflang and type attributes can be combined when specified with the alternate keyword.

The following example shows how you can specify versions of the page that use alternative formats, are aimed at other languages, and that are intended for other media:

<link rel=alternate href="/en/html" hreflang=en type=text/html title="English HTML">
<link rel=alternate href="/fr/html" hreflang=fr type=text/html title="French HTML">
<link rel=alternate href="/en/html/print" hreflang=en type=text/html media=print title="English HTML (for printing)">
<link rel=alternate href="/fr/html/print" hreflang=fr type=text/html media=print title="French HTML (for printing)">
<link rel=alternate href="/en/pdf" hreflang=en type=application/pdf title="English PDF">
<link rel=alternate href="/fr/pdf" hreflang=fr type=application/pdf title="French PDF">

This relationship is transitive — that is, if a document links to two other documents with the link type "alternate", then, in addition to implying that those documents are alternative representations of the first document, it is also implying that those two documents are alternative representations of each other.

The author keyword may be used with link, a, and area elements. This keyword creates a hyperlink.

For a and area elements, the author keyword indicates that the referenced document provides further information about the author of the nearest article element ancestor of the element defining the hyperlink, if there is one, or of the page as a whole, otherwise.

For link elements, the author keyword indicates that the referenced document provides further information about the author for the page as a whole.

The "referenced document" can be, and often is, a mailto: URL giving the email address of the author. [MAILTO]

Synonyms: For historical reasons, user agents must also treat link, a, and area elements that have a rev attribute with the value "made" as having the author keyword specified as a link relationship.

The bookmark keyword may be used with a and area elements. This keyword creates a hyperlink.

The bookmark keyword gives a permalink for the nearest ancestor article element of the linking element in question, or of the section the linking element is most closely associated with, if there are no ancestor article elements.

The following snippet has three permalinks. A user agent could determine which permalink applies to which part of the spec by looking at where the permalinks are given.

 ...
 <body>
  <h1>Example of permalinks</h1>
  <div id="a">
   <h2>First example</h2>
   <p><a href="a.html" rel="bookmark">This permalink applies to
   only the content from the first H2 to the second H2</a>. The DIV isn't
   exactly that section, but it roughly corresponds to it.</p>
  </div>
  <h2>Second example</h2>
  <article id="b">
   <p><a href="b.html" rel="bookmark">This permalink applies to
   the outer ARTICLE element</a> (which could be, e.g., a blog post).</p>
   <article id="c">
    <p><a href="c.html" rel="bookmark">This permalink applies to
    the inner ARTICLE element</a> (which could be, e.g., a blog comment).</p>
   </article>
  </article>
 </body>
 ...

The canonical keyword may be used with link element. This keyword creates a hyperlink.

The canonical keyword indicates that URL given by the href attribute is the preferred URL for the current document. That helps search engines reduce duplicate content, as described in more detail in The Canonical Link Relation. [RFC6596]

Link_types/dns-prefetch

Firefox3+Safari?Chrome46+
Opera?Edge79+
Edge (Legacy)NoInternet Explorer?
Firefox Android?Safari iOS?Chrome AndroidYesWebView Android46+Samsung Internet?Opera Android?

The dns-prefetch keyword may be used with link elements. This keyword creates an external resource link. This keyword is body-ok.

The dns-prefetch keyword indicates that preemptively performing DNS resolution for the origin of the specified resource is likely to be beneficial, as it is highly likely that the user will require resources located at that origin, and the user experience would be improved by preempting the latency costs associated with DNS resolution.

There is no default type for resources given by the dns-prefetch keyword.

The appropriate times to fetch and process this type of link are:

The fetch and process the linked resource steps for this type of linked resource, given a link element el, are:

  1. Let url be the result of encoding-parsing a URL given el's href attribute's value, relative to el's node document.

  2. If url is failure, then return.

  3. Let partitionKey be the result of determining the network partition key given el's node document's relevant settings object.

  4. The user agent should resolve an origin given partitionKey and url's origin.

    As the results of this algorithm can be cached, future fetches could be faster.

The expect keyword may be used with link elements. This keyword creates an internal resource link.

An internal resource link created by the expect keyword can be used to block rendering until the element that it indicates is connected to the document and fully parsed.

There is no default type for resources given by the expect keyword.

Whenever any of the following conditions occur for a link element el:

then process el.

To process internal resource link given a link element el, run these steps:

  1. Let doc be el's node document.

  2. Let url be the result of encoding-parsing a URL given el's href attribute's value, relative to doc.

  3. If this fails, or if url does not equal doc's URL with exclude fragments set to false, then unblock rendering on el and return.

  4. Let indicatedElement be the result of selecting the indicated part given doc and url.

  5. If all of the following are true:

    then block rendering on el.

  6. Otherwise, unblock rendering on el.

To process internal resource links given a Document doc:

  1. For each expect link element link in doc's render-blocking element set, process link.

The following attribute change steps, given element, localName, value, and namespace, are used to ensure expect link elements respond to dynamic id and name changes:

  1. If namespace is not null, then return.

  2. If element is in a stack of open elements of an HTML parser, then return.

  3. If any of the following is true:

    then process internal resource links given element's node document.

The external keyword may be used with a, area, and form elements. This keyword does not create a hyperlink, but annotates any other hyperlinks created by the element (the implied hyperlink, if no other keywords create one).

The external keyword indicates that the link is leading to a document that is not part of the site that the current document forms a part of.

The help keyword may be used with link, a, area, and form elements. This keyword creates a hyperlink.

For a, area, and form elements, the help keyword indicates that the referenced document provides further help information for the parent of the element defining the hyperlink, and its children.

In the following example, the form control has associated context-sensitive help. The user agent could use this information, for example, displaying the referenced document if the user presses the "Help" or "F1" key.

 <p><label> Topic: <input name=topic> <a href="help/topic.html" rel="help">(Help)</a></label></p>

For link elements, the help keyword indicates that the referenced document provides help for the page as a whole.

For a and area elements, on some browsers, the help keyword causes the link to use a different cursor.

4.6.7.9 Link type "icon"

Link_types#icon

Support in all current engines.

Firefox2+Safari3.1+Chrome4+
Opera9+Edge79+
Edge (Legacy)12+Internet Explorer11
Firefox Android4+Safari iOSNoChrome Android18+WebView Android38+Samsung Internet4.0+Opera AndroidNo
caniuse.com table

The icon keyword may be used with link elements. This keyword creates an external resource link.

The specified resource is an icon representing the page or site, and should be used by the user agent when representing the page in the user interface.

Icons could be auditory icons, visual icons, or other kinds of icons. If multiple icons are provided, the user agent must select the most appropriate icon according to the type, media, and sizes attributes. If there are multiple equally appropriate icons, user agents must use the last one declared in tree order at the time that the user agent collected the list of icons. If the user agent tries to use an icon but that icon is determined, upon closer examination, to in fact be inappropriate (e.g. because it uses an unsupported format), then the user agent must try the next-most-appropriate icon as determined by the attributes.

User agents are not required to update icons when the list of icons changes, but are encouraged to do so.

There is no default type for resources given by the icon keyword. However, for the purposes of determining the type of the resource, user agents must expect the resource to be an image.

The sizes keywords represent icon sizes in raw pixels (as opposed to CSS pixels).

An icon that is 50 CSS pixels wide intended for displays with a device pixel density of two device pixels per CSS pixel (2x, 192dpi) would have a width of 100 raw pixels. This feature does not support indicating that a different resource is to be used for small high-resolution icons vs large low-resolution icons (e.g. 50×50 2x vs 100×100 1x).

To parse and process the attribute's value, the user agent must first split the attribute's value on ASCII whitespace, and must then parse each resulting keyword to determine what it represents.

The any keyword represents that the resource contains a scalable icon, e.g. as provided by an SVG image.

Other keywords must be further parsed as follows to determine what they represent:

The keywords specified on the sizes attribute must not represent icon sizes that are not actually available in the linked resource.

The linked resource fetch setup steps for this type of linked resource, given a link element el and request request, are:

  1. Set request's destination to "image".

  2. Return true.

The process a link header steps for this type of linked resource are to do nothing.

In the absence of a link with the icon keyword, for Document objects whose URL's scheme is an HTTP(S) scheme, user agents may instead run these steps in parallel:

  1. Let request be a new request whose URL is the URL record obtained by resolving the URL "/favicon.ico" against the Document object's URL, client is the Document object's relevant settings object, destination is "image", synchronous flag is set, credentials mode is "include", and whose use-URL-credentials flag is set.

  2. Let response be the result of fetching request.

  3. Use response's unsafe response as an icon as if it had been declared using the icon keyword.

The following snippet shows the top part of an application with several icons.

<!DOCTYPE HTML>
<html lang="en">
 <head>
  <title>lsForums — Inbox</title>
  <link rel=icon href=favicon.png sizes="16x16" type="image/png">
  <link rel=icon href=windows.ico sizes="32x32 48x48" type="image/vnd.microsoft.icon">
  <link rel=icon href=mac.icns sizes="128x128 512x512 8192x8192 32768x32768">
  <link rel=icon href=iphone.png sizes="57x57" type="image/png">
  <link rel=icon href=gnome.svg sizes="any" type="image/svg+xml">
  <link rel=stylesheet href=lsforums.css>
  <script src=lsforums.js></script>
  <meta name=application-name content="lsForums">
 </head>
 <body>
  ...

For historical reasons, the icon keyword may be preceded by the keyword "shortcut". If the "shortcut" keyword is present, the rel attribute's entire value must be an ASCII case-insensitive match for the string "shortcut icon" (with a single U+0020 SPACE character between the tokens and no other ASCII whitespace).

The license keyword may be used with link, a, area, and form elements. This keyword creates a hyperlink.

The license keyword indicates that the referenced document provides the copyright license terms under which the main content of the current document is provided.

This specification does not specify how to distinguish between the main content of a document and content that is not deemed to be part of that main content. The distinction should be made clear to the user.

Consider a photo sharing site. A page on that site might describe and show a photograph, and the page might be marked up as follows:

<!DOCTYPE HTML>
<html lang="en">
 <head>
  <title>Exampl Pictures: Kissat</title>
  <link rel="stylesheet" href="/style/default">
 </head>
 <body>
  <h1>Kissat</h1>
  <nav>
   <a href="../">Return to photo index</a>
  </nav>
  <figure>
   <img src="/pix/39627052_fd8dcd98b5.jpg">
   <figcaption>Kissat</figcaption>
  </figure>
  <p>One of them has six toes!</p>
  <p><small><a rel="license" href="http://www.opensource.org/licenses/mit-license.php">MIT Licensed</a></small></p>
  <footer>
   <a href="/">Home</a> | <a href="../">Photo index</a>
   <p><small>© copyright 2009 Exampl Pictures. All Rights Reserved.</small></p>
  </footer>
 </body>
</html>

In this case the license applies to just the photo (the main content of the document), not the whole document. In particular not the design of the page itself, which is covered by the copyright given at the bottom of the document. This could be made clearer in the styling (e.g. making the license link prominently positioned near the photograph, while having the page copyright in light small text at the foot of the page).

Synonyms: For historical reasons, user agents must also treat the keyword "copyright" like the license keyword.

Link_types/manifest

Support in one engine only.

Firefox?Safari?ChromeNo
Opera?EdgeNo
Edge (Legacy)?Internet Explorer?
Firefox Android?Safari iOS?Chrome Android39+WebView Android?Samsung Internet?Opera Android?

The manifest keyword may be used with link elements. This keyword creates an external resource link.

The manifest keyword indicates the manifest file that provides metadata associated with the current document.

There is no default type for resources given by the manifest keyword.

When a web application is not installed, the appropriate time to fetch and process the linked resource for this link type is when the user agent deems it necessary. For example, when the user chooses to install the web application.

For an installed web application, the appropriate times to fetch and process the linked resource for this link type are:

In any case, only the first link element in tree order whose rel attribute contains the token manifest may be used.

A user agent must not delay the load event for this link type.

The linked resource fetch setup steps for this type of linked resource, given a link element el and request request, are:

  1. Let navigable be el's node document's node navigable.

  2. If navigable is null, then return false.

  3. If navigable is not a top-level traversable, then return false.

  4. Set request's initiator to "manifest".

  5. Set request's destination to "manifest".

  6. Set request's mode to "cors".

  7. Set request's credentials mode to the CORS settings attribute credentials mode for el's crossorigin content attribute.

  8. Return true.

To process this type of linked resource given a link element el, boolean success, response response, and byte sequence bodyBytes:

  1. If response's Content-Type metadata is not a JSON MIME type, then set success to false.

  2. If success is true:

    1. Let document URL be el's node document's URL.

    2. Let manifest URL be response's URL.

    3. Process the manifest given document URL, manifest URL, and bodyBytes. [MANIFEST]

The process a link header steps for this type of linked resource are to do nothing.

Link_types/modulepreload

Firefox115+Safari?Chrome66+
Opera?Edge79+
Edge (Legacy)NoInternet Explorer?
Firefox Android?Safari iOS?Chrome Android?WebView Android?Samsung Internet?Opera Android?

The modulepreload keyword may be used with link elements. This keyword creates an external resource link. This keyword is body-ok.

The modulepreload keyword is a specialized alternative to the preload keyword, with a processing model geared toward preloading module scripts. In particular, it uses the specific fetch behavior for module scripts (including, e.g., a different interpretation of the crossorigin attribute), and places the result into the appropriate module map for later evaluation. In contrast, a similar external resource link using the preload keyword would place the result in the preload cache, without affecting the document's module map.

Additionally, implementations can take advantage of the fact that module scripts declare their dependencies in order to fetch the specified module's dependency as well. This is intended as an optimization opportunity, since the user agent knows that, in all likelihood, those dependencies will also be needed later. It will not generally be observable without using technology such as service workers, or monitoring on the server side. Notably, the appropriate load or error events will occur after the specified module is fetched, and will not wait for any dependencies.

A user agent must not delay the load event for this link type.

The appropriate times to fetch and process the linked resource for such a link are:

Unlike some other link relations, changing the relevant attributes (such as as, crossorigin, and referrerpolicy) of such a link does not trigger a new fetch. This is because the document's module map has already been populated by a previous fetch, and so re-fetching would be pointless.

The fetch and process the linked resource algorithm for modulepreload links, given a link element el, is as follows:

  1. If el's href attribute's value is the empty string, then return.

  2. Let destination be the current state of el's as attribute (a destination), or "script" if it is in no state.

  3. If destination is not script-like, then queue an element task on the networking task source given el to fire an event named error at el, and return.

  4. Let url be the result of encoding-parsing a URL given el's href attribute's value, relative to el's node document.

  5. If url is failure, then return.

  6. Let settings object be el's node document's relevant settings object.

  7. Let credentials mode be the CORS settings attribute credentials mode for el's crossorigin attribute.

  8. Let cryptographic nonce be el.[[CryptographicNonce]].

  9. Let integrity metadata be the value of el's integrity attribute, if it is specified, or the empty string otherwise.

  10. Let referrer policy be the current state of el's referrerpolicy attribute.

  11. Let fetch priority be the current state of el's fetchpriority attribute.

  12. Let options be a script fetch options whose cryptographic nonce is cryptographic nonce, integrity metadata is integrity metadata, parser metadata is "not-parser-inserted", credentials mode is credentials mode, referrer policy is referrer policy, and fetch priority is fetch priority.

  13. Fetch a modulepreload module script graph given url, destination, settings object, options, and with the following steps given result:

    1. If result is null, then fire an event named error at el, and return.

    2. Fire an event named load at el.

The process a link header steps for this type of linked resource are to do nothing.

The following snippet shows the top part of an application with several modules preloaded:

<!DOCTYPE html>
<html lang="en">
<title>IRCFog</title>

<link rel="modulepreload" href="app.mjs">
<link rel="modulepreload" href="helpers.mjs">
<link rel="modulepreload" href="irc.mjs">
<link rel="modulepreload" href="fog-machine.mjs">

<script type="module" src="app.mjs">
...

Assume that the module graph for the application is as follows:

The module graph is rooted at app.mjs, which depends on irc.mjs and fog-machine.mjs. In turn, irc.mjs depends on helpers.mjs.

Here we see the application developer has used modulepreload to declare all of the modules in their module graph, ensuring that the user agent initiates fetches for them all. Without such preloading, the user agent might need to go through multiple network roundtrips before discovering helpers.mjs, if technologies such as HTTP/2 Server Push are not in play. In this way, modulepreload link elements can be used as a sort of "manifest" of the application's modules.

The following code shows how modulepreload links can be used in conjunction with import() to ensure network fetching is done ahead of time, so that when import() is called, the module is already ready (but not evaluated) in the module map:

<link rel="modulepreload" href="awesome-viewer.mjs">

<button onclick="import('./awesome-viewer.mjs').then(m => m.view())">
  View awesome thing
</button>

The nofollow keyword may be used with a, area, and form elements. This keyword does not create a hyperlink, but annotates any other hyperlinks created by the element (the implied hyperlink, if no other keywords create one).

The nofollow keyword indicates that the link is not endorsed by the original author or publisher of the page, or that the link to the referenced document was included primarily because of a commercial relationship between people affiliated with the two pages.

Link_types/noopener

Support in all current engines.

Firefox52+Safari10.1+Chrome49+
Opera?Edge79+
Edge (Legacy)?Internet ExplorerNo
Firefox Android?Safari iOS?Chrome Android?WebView Android?Samsung Internet?Opera Android?

Link_types/noopener

Support in all current engines.

Firefox52+Safari10.1+Chrome49+
Opera?Edge79+
Edge (Legacy)?Internet ExplorerNo
Firefox Android?Safari iOS?Chrome Android?WebView Android?Samsung Internet?Opera Android?

The noopener keyword may be used with a, area, and form elements. This keyword does not create a hyperlink, but annotates any other hyperlinks created by the element (the implied hyperlink, if no other keywords create one).

The keyword indicates that any newly created top-level traversable which results from following the hyperlink will not contain an auxiliary browsing context. E.g., the resulting Window's opener getter will return null.

See also the processing model.

This typically creates a top-level traversable with an auxiliary browsing context (assuming there is no existing navigable whose target name is "example"):

<a href=help.html target=example>Help!</a>

This creates a top-level traversable with a non-auxiliary browsing context (assuming the same thing):

<a href=help.html target=example rel=noopener>Help!</a>

These are equivalent and only navigate the parent navigable:

<a href=index.html target=_parent>Home</a>
<a href=index.html target=_parent rel=noopener>Home</a>

Link_types/noreferrer

Support in all current engines.

Firefox33+Safari5+Chrome16+
Opera?Edge79+
Edge (Legacy)13+Internet Explorer🔰 11
Firefox Android?Safari iOS?Chrome Android?WebView Android?Samsung Internet1.5+Opera Android?

Link_types/noreferrer

Support in all current engines.

Firefox33+Safari5+Chrome16+
Opera?Edge79+
Edge (Legacy)13+Internet Explorer🔰 11
Firefox Android?Safari iOS?Chrome Android?WebView Android?Samsung Internet1.5+Opera Android?

The noreferrer keyword may be used with a, area, and form elements. This keyword does not create a hyperlink, but annotates any other hyperlinks created by the element (the implied hyperlink, if no other keywords create one).

It indicates that no referrer information is to be leaked when following the link and also implies the noopener keyword behavior under the same conditions.

See also the processing model where referrer is directly manipulated.

<a href="..." rel="noreferrer" target="_blank"> has the same behavior as <a href="..." rel="noreferrer noopener" target="_blank">.

The opener keyword may be used with a, area, and form elements. This keyword does not create a hyperlink, but annotates any other hyperlinks created by the element (the implied hyperlink, if no other keywords create one).

The keyword indicates that any newly created top-level traversable which results from following the hyperlink will contain an auxiliary browsing context.

See also the processing model.

In the following example the opener is used to allow the help page popup to navigate its opener, e.g., in case what the user is looking for can be found elsewhere. An alternative might be to use a named target, rather than _blank, but this has the potential to clash with existing names.

<a href="..." rel=opener target=_blank>Help!</a>

The pingback keyword may be used with link elements. This keyword creates an external resource link. This keyword is body-ok.

For the semantics of the pingback keyword, see Pingback 1.0. [PINGBACK]

Link_types/preconnect

Support in all current engines.

Firefox39+Safari11.1+Chrome46+
Opera?Edge79+
Edge (Legacy)?Internet ExplorerNo
Firefox Android?Safari iOS?Chrome Android?WebView Android?Samsung Internet4.0+Opera Android?

The preconnect keyword may be used with link elements. This keyword creates an external resource link. This keyword is body-ok.

The preconnect keyword indicates that preemptively initiating a connection to the origin of the specified resource is likely to be beneficial, as it is highly likely that the user will require resources located at that origin, and the user experience would be improved by preempting the latency costs associated with establishing the connection.

There is no default type for resources given by the preconnect keyword.

A user agent must not delay the load event for this link type.

The appropriate times to fetch and process this type of link are:

The fetch and process the linked resource steps for this type of linked resource, given a link element el, are to create link options from el and to preconnect given the result.

The process a link header step for this type of linked resource given a link processing options options are to preconnect given options.

To preconnect given a link processing options options:

  1. If options's href is an empty string, returns.

  2. Let url be the result of encoding-parsing a URL given options's href, relative to options's base URL.

    Passing the base URL instead of a document or environment is tracked by issue #9715.

  3. If url is failure, then return.

  4. If url's scheme is not an HTTP(S) scheme, then return.

  5. Let partitionKey be the result of determining the network partition key given options's environment.

  6. Let useCredentials be true.

  7. If options's crossorigin is Anonymous and options's origin does not have the same origin as url's origin, then set useCredentials to false.

  8. The user agent should obtain a connection given partitionKey, url's origin, and useCredentials.

    This connection is obtained but not used directly. It will remain in the connection pool for subsequent use.

    The user agent should attempt to initiate a preconnect and perform the full connection handshake (DNS+TCP for HTTP, and DNS+TCP+TLS for HTTPS origins) whenever possible, but is allowed to elect to perform a partial handshake (DNS only for HTTP, and DNS or DNS+TCP for HTTPS origins), or skip it entirely, due to resource constraints or other reasons.

    The optimal number of connections per origin is dependent on the negotiated protocol, users current connectivity profile, available device resources, global connection limits, and other context specific variables. As a result, the decision for how many connections should be opened is deferred to the user agent.

Link_types/prefetch

Firefox2+SafariNoChrome8+
Opera?Edge79+
Edge (Legacy)12+Internet Explorer11
Firefox Android?Safari iOS?Chrome Android?WebView Android?Samsung Internet1.5+Opera Android?

The prefetch keyword may be used with link elements. This keyword creates an external resource link. This keyword is body-ok.

The prefetch keyword indicates that preemptively fetching and caching the specified resource or same-site document is likely to be beneficial, as it is highly likely that the user will require this resource for future navigations.

There is no default type for resources given by the prefetch keyword.

The appropriate times to fetch and process this type of link are:

The fetch and process the linked resource algorithm for prefetch links, given a link element el, is as follows:

  1. If el's href attribute's value is the empty string, then return.

  2. Let options be the result of creating link options from el.

  3. Set options's destination to the empty string.

  4. Let request be the result of creating a link request given options.

  5. If request is null, then return.

  6. Set request's initiator to "prefetch".

  7. Let processPrefetchResponse be the following steps given a response response and null, failure, or a byte sequence bytesOrNull:

    1. If response is a network error, fire an event named error at el.

    2. Otherwise, fire an event named load at el.

  8. The user agent should fetch request, with processResponseConsumeBody set to processPrefetchResponse. User agents may delay the fetching of request to prioritize other requests that are necessary for the current document.

The process a link header steps for this type of linked resource are to do nothing.

Link_types/preload

Support in one engine only.

Firefox85+Safari?Chrome🔰 50+
Opera37+Edge🔰 79+
Edge (Legacy)NoInternet Explorer?
Firefox Android?Safari iOS?Chrome Android?WebView Android50+Samsung Internet5.0+Opera Android?

The preload keyword may be used with link elements. This keyword creates an external resource link. This keyword is body-ok.

The preload keyword indicates that the user agent will preemptively fetch and cache the specified resource according to the potential destination given by the as attribute, and the priority given by the fetchpriority attribute, as it is highly likely that the user will require this resource for the current navigation.

User-agents might perform additional operations when a resource is loaded, such as preemptively decoding images or creating stylesheets. However, these additional operations cannot have observable effects.

There is no default type for resources given by the preload keyword.

A user agent must not delay the load event for this link type.

The appropriate times to fetch and process the linked resource for such a link are:

A Document has a map of preloaded resources, which is a map, initially empty.

A preload key is a struct. It has the following items:

URL
A URL
destination
A string
mode
A request mode, either "same-origin", "cors", or "no-cors"
credentials mode
A credentials mode

A preload entry is a struct. It has the following items:

integrity metadata
A string
response
Null or a response
on response available
Null, or an algorithm accepting a response or null

To consume a preloaded resource for Window window, given a URL url, a string destination, a string mode, a string credentialsMode, a string integrityMetadata, and onResponseAvailable, which is an algorithm accepting a response:

  1. Let key be a preload key whose URL is url, destination is destination, mode is mode, and credentials mode is credentialsMode.

  2. Let preloads be window's associated Document's map of preloaded resources.

  3. If key does not exist in preloads, then return false.

  4. Let entry be preloads[key].

  5. Let consumerIntegrityMetadata be the result of parsing integrityMetadata.

  6. Let preloadIntegrityMetadata be the result of parsing entry's integrity metadata.

  7. If none of the following condition apply:

    then return false.

    A mistmatch in integrity metadata between the preload and the consumer, even if both match the data, would lead to an additional fetch from the network.

    It is important that network errors are added to the preload cache so that if a preload request results in an error, the erroneous response isn't re-requested from the network later. This also has security implications; consider the case where a developer specifies subresource integrity metadata on a preload request, but not the following resource request. If the preload request fails subresource integrity verification and is discarded, the resource request will fetch and consume a potentially-malicious response from the network without verifying its integrity. [SRI]

  8. Remove preloads[key].

  9. If entry response is null, then set entry's on response available to onResponseAvailable.

  10. Otherwise, call onResponseAvailable with entry's response.

  11. Return true.

For the purposes of this section, a string type matches a string destination if the following algorithm returns true:

  1. If type is an empty string, then return true.

  2. If destination is "fetch", then return true.

  3. Let mimeTypeRecord be the result of parsing type.

  4. If mimeTypeRecord is failure, then return false.

  5. If mimeTypeRecord is not supported by the user agent, then return false.

  6. If any of the following are true:

    then return true.

  7. Return false.

To create a preload key for a request request, return a new preload key whose URL is request's URL, destination is request's destination, mode is request's mode, and credentials mode is request's credentials mode.

To preload given a link processing options options and an optional processResponse, which is an algorithm accepting a response:

  1. If options's type doesn't match options's destination, then return.

  2. If options's destination is "image" and options's source set is not null, then set options's href to the result of selecting an image source from options's source set.

  3. Let request be the result of creating a link request given options.

  4. If request is null, then return.

  5. Let unsafeEndTime be 0.

  6. Let entry be a new preload entry whose integrity metadata is options's integrity.

  7. Let key be the result of creating a preload key given request.

  8. If options's document is "pending", then set request's initiator type to "early hint".

  9. Let controller be null.

  10. Let reportTiming given a Document document be to report timing for controller given document's relevant global object.

  11. Set controller to the result of fetching request, with processResponseConsumeBody set to the following steps given a response response and null, failure, or a byte sequence bodyBytes:

    1. If bodyBytes is a byte sequence, then set response's body to bodyBytes as a body.

      By using processResponseConsumeBody, we have extracted the entire body. This is necessary to ensure the preloader loads the entire body from the network, regardless of whether the preload will be consumed (which is uncertain at this point). This step then resets the request's body to a new body containing the same bytes, so that other specifications can read from it at the time of actual consumption, despite us having already done so once.

    2. Otherwise, set response to a network error.

    3. Set unsafeEndTime to the unsafe shared current time.

    4. If options's document is not null, then call reportTiming given options's document.

    5. If entry's on response available is null, then set entry's response to response; otherwise call entry's on response available given response.

    6. If processResponse is given, then call processResponse with response.

  12. Let commit be the following steps given a Document document:

    1. If entry's response is not null, then call reportTiming given document.

    2. Set document's map of preloaded resources[key] to entry.

  13. If options's document is null, then set options's on document ready to commit. Otherwise call commit with options's document.

The fetch and process the linked resource steps for this type of linked resource, given a link element el, are:

  1. Update the source set for el.

  2. Let options be the result of creating link options from el.

  3. Preload options, with the following steps given a response response:

    1. If response is a network error, fire an event named error at el. Otherwise, fire an event named load at el.

      The actual browsers' behavior is different from the spec here, and the feasibility of changing the behavior has not yet been investigated. See issue #1142.

The process a link header step for this type of link given a link processing options options is to preload options.

The privacy-policy keyword may be used with link, a, and area elements. This keyword creates a hyperlink.

The privacy-policy keyword indicates that the referenced document contains information about the data collection and usage practices that apply to the current document, as described in more detail in Additional Link Relation Types. The referenced document may be a standalone privacy policy, or a specific section of some more general document. [RFC6903]

The search keyword may be used with link, a, area, and form elements. This keyword creates a hyperlink.

The search keyword indicates that the referenced document provides an interface specifically for searching the document and its related resources.

OpenSearch description documents can be used with link elements and the search link type to enable user agents to autodiscover search interfaces. [OPENSEARCH]

The stylesheet keyword may be used with link elements. This keyword creates an external resource link that contributes to the styling processing model. This keyword is body-ok.

The specified resource is a CSS style sheet that describes how to present the document.

Alternative_style_sheets

Support in one engine only.

Firefox3+Safari?Chrome1–48
OperaYesEdgeNo
Edge (Legacy)?Internet Explorer8+
Firefox Android?Safari iOS?Chrome Android?WebView Android?Samsung Internet?Opera Android?

If the alternate keyword is also specified on the link element, then the link is an alternative style sheet; in this case, the title attribute must be specified on the link element, with a non-empty value.

The default type for resources given by the stylesheet keyword is text/css.

A link element of this type is implicitly potentially render-blocking if the element was created by its node document's parser.

When the disabled attribute of a link element with a stylesheet keyword is set, disable the associated CSS style sheet.

The appropriate times to fetch and process this type of link are:

Quirk: If the document has been set to quirks mode, has the same origin as the URL of the external resource, and the Content-Type metadata of the external resource is not a supported style sheet type, the user agent must instead assume it to be text/css.

The linked resource fetch setup steps for this type of linked resource, given a link element el and request request, are:

  1. If el's disabled attribute is set, then return false.

  2. If el contributes a script-blocking style sheet, append el to its node document's script-blocking style sheet set.

  3. If el's media attribute's value matches the environment and el is potentially render-blocking, then block rendering on el.

  4. If el is currently render-blocking, then set request's render-blocking to true.

  5. Return true.

See issue #968 for plans to use the CSSOM fetch a CSS style sheet algorithm instead of the default fetch and process the linked resource algorithm. In the meantime, any critical subresource request should have its render-blocking set to whether or not the link element is currently render-blocking.

To process this type of linked resource given a link element el, boolean success, response response, and byte sequence bodyBytes:

  1. If the resource's Content-Type metadata is not text/css, then set success to false.

  2. If el no longer creates an external resource link that contributes to the styling processing model, or if, since the resource in question was fetched, it has become appropriate to fetch it again, then:

    1. Remove el from el's node document's script-blocking style sheet set.

    2. Return.

  3. If el has an associated CSS style sheet, remove the CSS style sheet.

  4. If success is true, then:

    1. Create a CSS style sheet with the following properties:

      type

      text/css

      location

      response's URL list[0]

      We provide a URL here on the assumption that w3c/csswg-drafts issue #9316 will be fixed.

      owner node

      el

      media

      The media attribute of el.

      This is a reference to the (possibly absent at this time) attribute, rather than a copy of the attribute's current value. CSSOM defines what happens when the attribute is dynamically set, changed, or removed.

      title

      The title attribute of el, if el is in a document tree, or the empty string otherwise.

      This is similarly a reference to the attribute, rather than a copy of the attribute's current value.

      alternate flag

      Set if the link is an alternative style sheet and el's explicitly enabled is false; unset otherwise.

      origin-clean flag

      Set if the resource is CORS-same-origin; unset otherwise.

      parent CSS style sheet
      owner CSS rule

      null

      disabled flag

      Left at its default value.

      CSS rules

      Left uninitialized.

      This doesn't seem right. Presumably we should be using bodyBytes? Tracked as issue #2997.

      The CSS environment encoding is the result of running the following steps: [CSSSYNTAX]

      1. If el has a charset attribute, get an encoding from that attribute's value. If that succeeds, return the resulting encoding. [ENCODING]

      2. Otherwise, return the document's character encoding. [DOM]

    2. Fire an event named load at el.

  5. Otherwise, fire an event named error at el.

  6. If el contributes a script-blocking style sheet, then:

    1. Assert: el's node document's script-blocking style sheet set contains el.

    2. Remove el from its node document's script-blocking style sheet set.

  7. Unblock rendering on el.

The process a link header steps for this type of linked resource are to do nothing.

The tag keyword may be used with a and area elements. This keyword creates a hyperlink.

The tag keyword indicates that the tag that the referenced document represents applies to the current document.

Since it indicates that the tag applies to the current document, it would be inappropriate to use this keyword in the markup of a tag cloud, which lists the popular tags across a set of pages.

This document is about some gems, and so it is tagged with "https://en.wikipedia.org/wiki/Gemstone" to unambiguously categorize it as applying to the "jewel" kind of gems, and not to, say, the towns in the US, the Ruby package format, or the Swiss locomotive class:

<!DOCTYPE HTML>
<html lang="en">
 <head>
  <title>My Precious</title>
 </head>
 <body>
  <header><h1>My precious</h1> <p>Summer 2012</p></header>
  <p>Recently I managed to dispose of a red gem that had been
  bothering me. I now have a much nicer blue sapphire.</p>
  <p>The red gem had been found in a bauxite stone while I was digging
  out the office level, but nobody was willing to haul it away. The
  same red gem stayed there for literally years.</p>
  <footer>
   Tags: <a rel=tag href="https://en.wikipedia.org/wiki/Gemstone">Gemstone</a>
  </footer>
 </body>
</html>

In this document, there are two articles. The "tag" link, however, applies to the whole page (and would do so wherever it was placed, including if it was within the article elements).

<!DOCTYPE HTML>
<html lang="en">
 <head>
  <title>Gem 4/4</title>
 </head>
 <body>
  <article>
   <h1>801: Steinbock</h1>
   <p>The number 801 Gem 4/4 electro-diesel has an ibex and was rebuilt in 2002.</p>
  </article>
  <article>
   <h1>802: Murmeltier</h1>
   <figure>
    <img src="https://upload.wikimedia.org/wikipedia/commons/b/b0/Trains_de_la_Bernina_en_hiver_2.jpg"
         alt="The 802 was red with pantographs and tall vents on the side.">
    <figcaption>The 802 in the 1980s, above Lago Bianco.</figcaption>
   </figure>
   <p>The number 802 Gem 4/4 electro-diesel has a marmot and was rebuilt in 2003.</p>
  </article>
  <p class="topic"><a rel=tag href="https://en.wikipedia.org/wiki/Rhaetian_Railway_Gem_4/4">Gem 4/4</a></p>
 </body>
</html>

The terms-of-service keyword may be used with link, a, and area elements. This keyword creates a hyperlink.

The terms-of-service keyword indicates that the referenced document contains information about the agreements between the current document's provider and users who wish to use the current document, as described in more detail in Additional Link Relation Types. [RFC6903]

Some documents form part of a sequence of documents.

A sequence of documents is one where each document can have a previous sibling and a next sibling. A document with no previous sibling is the start of its sequence, a document with no next sibling is the end of its sequence.

A document may be part of multiple sequences.

The next keyword may be used with link, a, area, and form elements. This keyword creates a hyperlink.

The next keyword indicates that the document is part of a sequence, and that the link is leading to the document that is the next logical document in the sequence.

When the next keyword is used with a link element, user agents should process such links as if they were using one of the dns-prefetch, preconnect, or prefetch keywords. Which keyword the user agent wishes to use is implementation-dependent; for example, a user agent may wish to use the less-costly preconnect processing model when trying to conserve data, battery power, or processing power, or may wish to pick a keyword depending on heuristic analysis of past user behavior in similar scenarios.

The prev keyword may be used with link, a, area, and form elements. This keyword creates a hyperlink.

The prev keyword indicates that the document is part of a sequence, and that the link is leading to the document that is the previous logical document in the sequence.

Synonyms: For historical reasons, user agents must also treat the keyword "previous" like the prev keyword.

Extensions to the predefined set of link types may be registered on the microformats page for existing rel values. [MFREL]

Anyone is free to edit the microformats page for existing rel values at any time to add a type. Extension types must be specified with the following information:

Keyword

The actual value being defined. The value should not be confusingly similar to any other defined value (e.g. differing only in case).

If the value contains a U+003A COLON character (:), it must also be an absolute URL.

Effect on... link

One of the following:

Not allowed
The keyword must not be specified on link elements.
Hyperlink
The keyword may be specified on a link element; it creates a hyperlink.
External Resource
The keyword may be specified on a link element; it creates an external resource link.
Effect on... a and area

One of the following:

Not allowed
The keyword must not be specified on a and area elements.
Hyperlink
The keyword may be specified on a and area elements; it creates a hyperlink.
External Resource
The keyword may be specified on a and area elements; it creates an external resource link.
Hyperlink Annotation
The keyword may be specified on a and area elements; it annotates other hyperlinks created by the element.
Effect on... form

One of the following:

Not allowed
The keyword must not be specified on form elements.
Hyperlink
The keyword may be specified on form elements; it creates a hyperlink.
External Resource
The keyword may be specified on form elements; it creates an external resource link.
Hyperlink Annotation
The keyword may be specified on form elements; it annotates other hyperlinks created by the element.
Brief description

A short non-normative description of what the keyword's meaning is.

Specification

A link to a more detailed description of the keyword's semantics and requirements. It could be another page on the wiki, or a link to an external page.

Synonyms

A list of other keyword values that have exactly the same processing requirements. Authors should not use the values defined to be synonyms, they are only intended to allow user agents to support legacy content. Anyone may remove synonyms that are not used in practice; only names that need to be processed as synonyms for compatibility with legacy content are to be registered in this way.

Status

One of the following:

Proposed
The keyword has not received wide peer review and approval. Someone has proposed it and is, or soon will be, using it.
Ratified
The keyword has received wide peer review and approval. It has a specification that unambiguously defines how to handle pages that use the keyword, including when they use it in incorrect ways.
Discontinued
The keyword has received wide peer review and it has been found wanting. Existing pages are using this keyword, but new pages should avoid it. The "brief description" and "specification" entries will give details of what authors should use instead, if anything.

If a keyword is found to be redundant with existing values, it should be removed and listed as a synonym for the existing value.

If a keyword is registered in the "proposed" state for a period of a month or more without being used or specified, then it may be removed from the registry.

If a keyword is added with the "proposed" status and found to be redundant with existing values, it should be removed and listed as a synonym for the existing value. If a keyword is added with the "proposed" status and found to be harmful, then it should be changed to "discontinued" status.

Anyone can change the status at any time, but should only do so in accordance with the definitions above.

Conformance checkers must use the information given on the microformats page for existing rel values to establish if a value is allowed or not: values defined in this specification or marked as "proposed" or "ratified" must be accepted when used on the elements for which they apply as described in the "Effect on..." field, whereas values marked as "discontinued" or not listed in either this specification or on the aforementioned page must be rejected as invalid. Conformance checkers may cache this information (e.g. for performance reasons or to avoid the use of unreliable network connectivity).

When an author uses a new type not defined by either this specification or the wiki page, conformance checkers should offer to add the value to the wiki, with the details described above, with the "proposed" status.

Types defined as extensions in the microformats page for existing rel values with the status "proposed" or "ratified" may be used with the rel attribute on link, a, and area elements in accordance to the "Effect on..." field. [MFREL]