This document explains some common mistakes in user agents due to incorrect or incomplete implementation of specifications, and suggests remedies. It also suggests some "good behavior" where specifications themselves do not specify any particular behavior (e.g., in the face of error conditions). This document is not a complete set of guidelines for good user agent behavior.
Note: This document does not incriminate specific user agents. W3C does not generally track bugs or errors in implementations. That information is generally tracked by the vendors themselves or third parties.
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This document explains some common mistakes in user agents due to incorrect or incomplete implementation of specifications, and suggests remedies. It also suggests "good behavior" where specifications themselves do not specify any particular behavior (e.g., in the face of error conditions).
Each suggestion in this document, called a "checkpoint" begins with a one-sentence description of the "right thing to do." Checkpoints also include rationale, examples, and references. Checkpoints are not ranked according to importance. They are not listed in any particular order.
This document does not address accessibility issues for user agents. Please refer to W3C's User Agent Accessibility Guidelines 1.0 [UAAG10] for information on how to design user agents that are accessible to people with disabilities.
This section focuses on the user's experience, including customization, user interface, and other usability issues.
There are many ways to indicate to the user that a link is broken. The recommended behavior is as follows:
Wrong: Some user agents scroll to the top or bottom of the document when the user attempts to follow a broken link. This behavior is discouraged since it is indistinguishable from the correct behavior when a target is at the beginning or end of a document.
User agents may not be able to render certain types of content on the Web either natively or through a plug-in (e.g., XML content, XSLT style sheets, RDF documents, DTDs, XML schemas, etc). User agents should allow users to retrieve and save these resources, otherwise users may not be able to access this Web content at all.
The presentation of the frameset could be achieved, for example, by:
Note: The authors do not encourage Web content developers to use frames as they can cause many usability and accessibility problems.
For instance, allow users to associate external programs with URI schemes. The user agent should inform the user when it does not recognize a URI scheme in content.
A user may want the "tel" scheme (e.g.,
tel:+33-4-12-34) to interact with their
telephone. Or they may want the "irc" scheme (e.g.,
irc://irc.example.org/) to activate an IRC client on
their desktop with a connection to the specified server.
Wrong: Some user agents ignore the scheme part (before the ":") when the scheme is unknown to them, interpret the colon character as though it were encoded as '%3A' and then treat the URI as though it were a relative URI, usually producing a broken link (and confusing users).
An absolute URI contains the name of the scheme being used followed by a colon (":") and then a string whose interpretation depends on the scheme.
Many user agents compensate for incomplete URIs by applying a
series of transformations with the hope of creating a URI that
works. For example, many user agents transform the string
www.w3.org into the URI
user should be able to control whether, for example, typing a
keyword should invoke a Web search or whether the user agent
http://www. and append
Rendering an incomplete document as though it were complete is very likely to confuse users. Part of the document is missing, hence some anchors might not be present, possibly breaking some links. The user agent should notify the user that the document is incomplete.
The HTTP/1.1 specification describes this behavior for caches at the protocol level. Partial responses should also be made obvious to the user with a warning.
A cache MUST NOT return a partial response to a client without explicitly marking it as such, using the 206 (Partial Content) status code. A cache MUST NOT return a partial response using a status code of 200 (OK).
Many browsers allow configuration to save HTTP authentication [RFC2616, RFC2617] information ("remember my password"). They should also allow users to "flush" that authentication information on request. For instance, the user may wish to leave the user agent running but tell it to forget the password to access the user's bank account.
Wrong: Most user agents consider that authentication information (e.g., password) provided by a user for a server/realm pair during a session is immutable for the duration of the session.
Metadata – data about data – can provide very useful context
to users about information on the Web. For instance, metadata about
a book might include the book's author, title, publication date,
publisher, etc. (refer to the Dublin Core [DC] for information about library-type
metadata). Authors include metadata in HTML documents through a
variety of elements and attributes (e.g., the
ADDRESS elements, the "alt", "title", and
"summary" attributes, etc. Languages such as the Resource
Description Framework [RDF] allow users to
populate the Web with rich metadata. User agents should provide a user
interface to allow users to view metadata. The user interface may
vary according to the underlying markup language. For instance,
many graphical browsers render the HTML "title" attribute (e.g., as
a tool-tip) when the user selects or hovers over an element with
that attribute specified.
Users may wish to track and archive HTTP POST requests for the same reasons they wish to track and archive email. For instance, if the user places a book order through a form, and that form uses a POST request, the user should be able to store information about that transaction.
The HTTP/1.1 protocol [RFC2616] allows the client to request a representation of a resource which is best suited to its needs (language, media type, etc); this mechanism is called "content negotiation".
When a resource is negotiated, the user might want to bookmark a particular version. For example, a document might be available in several languages under the same URI, and the user might want to point somebody to the Canadian version of this document, which has a different URI.
In such a case, it should be possible to bookmark either the original URI or the URI of the view that the user got. The original URI can be interpreted as being the generic object and the retrieved document as one view of this object.
HTTP/1.1 [RFC2616] allows transfer encoding. An example of encoding is data compression, which speeds up Web browsing over a slow connection.
The user agent should allow the user to set the transfer encoding in the HTTP requests sent out.
The user should be allowed to specify the set of languages that the user agent may use for language negotiation.
In case the user does not specify any language, the user agent may use the language of its user interface as the value sent out. The user agent should allow the user to override this behavior.
Accept-Languageheader, see section 14.4 of the HTTP/1.1 specification, [RFC2616].
Accept-Languageheader, see section 15.1.4 of the HTTP/1.1 specification, [RFC2616].
This section focuses on issues related to style sheets and link types.
A style sheet is a set of rules that specifies how to render a document on a graphical desktop computer monitor, on paper, as synthesized speech, etc. A document may have more than one style sheet associated with it, and users should be able to select from alternative style sheets.
Some markup and style sheet languages allow authors
@media construct in [CSS2],
media attribute in [HTML 4.01]) to design
documents that are rendered differently according to the
characteristics of the output device: whether graphical display,
television screen, handheld device, speech synthesizer, braille
Users must be able to view content even without style sheets.
Wrong: In some user agents, missing style sheets result in a fatal error or result in the user agent not rendering content.
For each source document, [a user agent] must attempt to retrieve all associated style sheets that are appropriate for the supported media types. If it cannot retrieve all associated style sheets (for instance, because of network errors), it must display the document using those it can retrieve.
6.12 of the HTML 4.01 Recommendation [HTML 4.01] lists some link types that may
be used by authors to make assertions about linked Web
resources. These include
glossary, and others.
Although the HTML 4.01 specification does not specify definitive
rendering or behavior for these link types, user agents should
interpret them in useful ways. For instance, the
types may be used to build a table of contents, or may be used to
identify the print order of documents, etc.
This section focuses on the implementation of network protocols used to download resources from the Web.
The media type of a resource retrieved by HTTP [RFC2616] is determined by the content type and encoding returned by the server in the response headers.
If the user wants to save a resource locally, the user agent should
respect the system naming conventions for files (e.g. PNG images
usually have a
is a view of the gzip'ed PostScript version of the HTML 4.01
specification. The HTTP headers sent by the server include:
Content-Type: application/postscript; qs=0.001 Content-Encoding: gzip
If saved locally, the filename on most computers should be
html40.ps.gz for the applications to recognize the file
Wrong: Saving this compressed PostScript document
html40.ps is likely to confuse other applications.
If an HTML document is returned with a
text/plain, the user agent must render the
document as plain text without interpreting HTML elements and
attributes (i.e. the HTML source must be displayed).
User agents must respect the character set when it is explicitly
specified in the response. The character set can be given by the HTTP
Content-Type headers and/or by the document-internal
meta element, etc).
HTTP/1.1 recipients MUST respect the charset label provided by the sender; and those user agents that have a provision to "guess" a charset MUST use the charset from the
content-typefield if they support that charset [..].
To sum up, conforming user agents must observe the following priorities when determining a document's character encoding (from highest priority to lowest):
- An HTTP "charset" parameter in a "
- A META declaration with "
http-equiv" set to "
Content-Type" and a value set for "charset".
- The charset attribute set on an element that designates an external resource.
The HTTP/1.1 specification [RFC2616] specifies several types of redirects. The two most common are designated by the codes 301 (permanent) and 302 or 307 (temporary):
Wrong: User agents usually show the user (in the user interface) the URI that is the result of a temporary (302 or 307) redirect, as they would do for a permanent (301) redirect.
Many Web sites have a single hostname like www.example.org resolve to multiple servers for the purpose of load balancing or mirroring. If one server is unreachable, others may still be up, so browsers should try to contact all the servers of a Web site before concluding that the Web site is down.
HTTP/1.1 [RFC2616] defines content negotiation. The client sending out a request gives a list of media types that it is willing to accept; the server then returns a representation of the object requested in one of the specified formats if it is available.
When entities are embedded in a document (such as images in HTML
documents), user agents should only send
for the formats they support.
If a user agent can render JPEG, PNG and GIF images, the list of
media types accepted should be
Wrong: User agent agents should not send an HTTP
Accept: */* since the server may support
content types that the user agent does not. For instance, if a server
is configured so that SVG images are preferred to PNG images, a user
agent that only supports PNG, GIF, and JPEG will receive (unsupported)
SVG rather than (supported) PNG.
Resources are located on the Web using Uniform Resources Identifiers [RFC2396]. This section discusses how user agents should handle URIs.
When a resource (
URI1) has moved, an HTTP redirect
can indicate its new location (
URI1 has a fragment identifier
then the new target that the user agent should be trying to reach
URI2 already has a
fragment identifier, then
#frag must not be appended and
the new target is
Wrong: Most current user agents do implement HTTP redirects but do not append the fragment identifier to the new URI, which generally confuses the user because they end up with the wrong resource.
Suppose that a user requests the resource at
http://www.w3.org/TR/WD-ruby/#changes and the server
redirects the user agent to
Before fetching that latter URI, the browser should append the fragment
#changes to it:
The authors would like to thank the W3C Team for their input.