Publishing and Linking on the Web

W3C Editor's Draft 04 January 2012

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Jeni Tennison, Independent
Daniel Appelquist, Telefónica


The Web borrows concepts from physical media (the notion of a "page," for example) and overlays them on top of a networked infrastructure (the Internet) and digital presentation medium (the browser software). This mapping is an abstraction to enable the Web user to more easily interact with content and applications. However, when social or legal concepts and frameworks relating documents, publishing and speech are applied to the Web, this abstraction often does not suffice. Publishing a page on the Web is actually fundamentally different from printing and distributing a page in a magazine or book but because the social conventions around these physical media are so strong and have been reinforced through our society for hundreds of years, it is all too tempting to try to apply them to the Web when in fact this application may not be appropriate.

This document was written, in part, because of some legal issues that were raised to the TAG. It does not attempt to answer these legal questions, but rather it seeks to set definitions for terms which could inform future social and legal dialog and opinion around publishing and linking on the Web.

Status of This Document

This section describes the status of this document at the time of its publication. Other documents may supersede this document. A list of current W3C publications and the latest revision of this technical report can be found in the W3C technical reports index at http://www.w3.org/TR/.

This is an Editor's Draft which the TAG intends to become a Working Draft on the Recommendation track at W3C. The previous version of this document was published as a Draft TAG Finding.

This document was published by the Technical Architecture Group as an Editor's Draft. If you wish to make comments regarding this document, please send them to www-tag@w3.org (subscribe, archives). All feedback is welcome.

Publication as an Editor's Draft does not imply endorsement by the W3C Membership. This is a draft document and may be updated, replaced or obsoleted by other documents at any time. It is inappropriate to cite this document as other than work in progress.

This document was produced by a group operating under the 5 February 2004 W3C Patent Policy. W3C maintains a public list of any patent disclosures made in connection with the deliverables of the group; that page also includes instructions for disclosing a patent. An individual who has actual knowledge of a patent which the individual believes contains Essential Claim(s) must disclose the information in accordance with section 6 of the W3C Patent Policy.

Table of Contents

1. Introduction

The act of viewing a web page is a complex interaction between a user's browser and any number of web servers. Unlike reading a book, viewing a web page is an act of copying: the data held on the servers is copied onto the user's computer. The page itself may cause more copying to take place — of images, videos and other files, perhaps from other servers, that are displayed or otherwise used within the original page — without the user's explicit knowledge or consent. For an end user, it is usually impossible to tell whether a given image or video displayed within a page originates from the server the page comes from or from some other location.

Proxy servers and services that combine and repackage data from other sources may also retain copies of this material, due to the user's original request for the page. These intermediary services may transform, translate or rewrite some of the material that passes through them, to enhance the user's experience of the web page.

Still other services on the web, such as search engines and archives, make copies of content as a matter of course, to provide value to their users and to the original authors of the web page (as it enables the content to be found more easily).

1.1 Background

Against this complex backdrop, many content publishers and other entities have sought to control the use of their content on the Web. In some cases, they have employed means that do not take into account the Web's inherent complexities, and they have not used technical mechanisms available to them. Some illustrative (but by no means comprehensive) examples of this are provided as background.

Licenses that describe how material may be copied and altered by others tend not to take account of this complexity, for example to distinguish between a proxy compressing a web page to make it load faster and someone editing and republishing the page on their own website. To illustrate, the Creative Commons Attribution-NoDerivs defines the terms:

means a work based upon the Work, or upon the Work and other pre-existing works, such as a translation, adaptation, derivative work, arrangement of music or other alterations of a literary or artistic work, or phonogram or performance and includes cinematographic adaptations or any other form in which the Work may be recast, transformed, or adapted including in any form recognizably derived from the original, except that a work that constitutes a Collection will not be considered an Adaptation for the purpose of this License. For the avoidance of doubt, where the Work is a musical work, performance or phonogram, the synchronization of the Work in timed-relation with a moving image ("synching") will be considered an Adaptation for the purpose of this License.
means to make available to the public the original and copies of the Work through sale or other transfer of ownership.
means to make copies of the Work by any means including without limitation by sound or visual recordings and the right of fixation and reproducing fixations of the Work, including storage of a protected performance or phonogram in digital form or other electronic medium.

Terms and Conditions statements on websites also list acceptable and unacceptable behaviour on a site, with any browsing on the site implicitly indicating acceptance of the terms. These generally do not take into account the behaviour of proxies that transform content on-the-fly for mobile consumption, for example. For instance, one standard set of terms and conditions includes:

You may view, download for caching purposes only, and print pages from the website for your own personal use, subject to the restrictions set out below and elsewhere in these terms of use.

You must not:

(a) republish material from this website (including republication on another website);

(b) sell, rent or sub-license material from the website;

(c) show any material from the website in public;

(d) reproduce, duplicate, copy or otherwise exploit material on our website for a commercial purpose;

(e) edit or otherwise modify any material on the website; or

(f) redistribute material from this website except for content specifically and expressly made available for redistribution (such as our newsletter)

Limits placed on the use of a website often include limitations on automatic indexing of the website, without exceptions for the search engines that make the website discoverable or the archives that ensure its longevity. For example, the same set of terms and conditions as described above includes:

You must not conduct any systematic or automated data collection activities (including without limitation scraping, data mining, data extraction and data harvesting) on or in relation to our website without our express written consent.

As another example, the terms and conditions for gsig.com include:

Use of Materials: Upon your agreement to the Terms, GSI grants you the right to view the site and to download materials from this site for your personal, non-commercial use. You are not authorized to use the materials for any other purpose. If you do download or otherwise reproduce the materials from this Site, you must reproduce all of GSI’s proprietary markings, such as copyright and trademark notices, in the same form and manner as the original.


You may not use any “deep-link”, “page-scrape”, “robot”, “spider” or any other automatic device, program, algorithm or methodology or any similar or equivalent manual process to access, acquire, copy or monitor any portion of the Site or any of its content, or in any way reproduce or circumvent the navigational structure or presentation of the Site.

Many sites have a linking policy that limits what links can be made to the site from other sites; that these policies are directly accessible through searches for "all links should point to" illustrates that these conditions are not backed up through technical mechanisms that would prevent such links from being made. For example, the website at quotec.co.uk has a linking policy that includes:

Links pointing to this website should not be misleading.

Appropriate link text should be always be used.

From time to time we may update the URL structure of our website, and unless we agree in writing otherwise, all links should point to http://www.quotec.co.uk.

You must not use our logo to link to this website (or otherwise) without our express written permission.

You must not link to this website using any inline linking technique.

You must not frame the content of this website or use any similar technology in relation to the content of this website.

Legislation that governs the possession and distribution of illegal material (such as child pornography, information that is under copyright or material that is legally suppressed through a gag order) often needs to exempt certain types of services, such as caching or hosting, as it would be impractical for the people running those services to police all the material that passes through their servers. An example of legislation that does this in the UK is the Coroners and Justice Act 2009 Schedule 13; from the Explanatory Notes (emphasis added):

Paragraphs 3 to 5 of [Schedule 13] provide exemptions for internet service providers from the offence of possession of prohibited images of children in limited circumstances, such as where they are acting as mere conduits for such material or are storing it as caches or hosts.

Examples of the kind of legal questions that have arisen are:

There are many other examples on the Wikipedia page on Copyright aspects of hyperlinking and framing.

1.2 Scope of this Document

This document does not aim to address whether particular activities on the web are illegal or legal; this is outside the scope of the TAG. Instead, it aims to:

2. Terminology

This section summarises the terminology that is used within this paper. More details about each of the terms is given in the rest of the document.

2.1 Actors and Agents

Archiving Server
A service that stores copies of web content, e.g. to provide an ongoing historical record
A user agent that fetches and displays web pages for users.
Caching Server
An intermediary server that stores a copy of web content from an origin server.
A company, organization or individual who manages web content on a web server and determines what it serves.
A user agent which retrieves web content automatically
Hosting Server
A web server that hosts web content.
Intermediary Server
A web server that provides access to web content that originates from elsewhere on the web.
Origin Server
In the case where an intermediary server relays web content, the web server from which the content originates.
Proxy Server
A kind of intermediary server that plays a relaying role between web server and user agent
A service that reuses web content from elsewhere on the web, adding value to it by combining it with other information or reformatting it, e.g. a “mashup.”
Search Engine
A service that indexes web content and provides an interface to search this index.
Service Provider
A company, organization or individual who owns and operates a web server.
A real person. Somebody who is using a browser.
User Agent
Any piece of software that accesses web content.
Web Server
A piece of software that makes content available on the web.

2.2 Artifacts

An alternative address for a piece of web content that redirects requests to the main address
A pointer to the address of a file within a web page
For the purposes of this document, any file available on the Web, such as images, video, style sheets and scripts. **
Web Content
Sets of Resources or Web Documents.
Web Document
A file, usually HTML, which is served from a Web server and consumed by a Web browser, usually resulting in a User viewing a Web Page.
Web Page
What the user sees when they load a Web document and its associated resources.

2.3 Actions

To permanently store a copy of a file that is hosted by another server
To store a copy of a file that is hosted by another server which is updated when the original file changes
In the context of this document, the duplication of bits from one place to another place. Since copy is an imprecise term, we have tried to use more specific terms such as cache, distribute, alias and include.
To provide access to a file hosted by another server without keeping a copy
To include a Resource (such as an Image or a Video) within a Web Page such that it is visible to a User
To store a file and provide access to it on the web
To use another file, such as an image or a script, within a web page
To extract information from a file that is hosted by another server in a structure that improves retrieval of that file or a link to the file
To change a resource such as by reformatting, resizing or rewriting a document
To put a file on a server such that it is given an address on the web

3. Publishing

The concept of publishing on the web has evolved as the web's ecosystem has enlarged and diversified, and as the capabilities of browsers and the web standards that they implement have developed. There is no single definition of what publishing on the web means. Instead there are a number of activities that could be viewed as publication or distribution in a legal sense, or something else. This section describes each of these activities and how they work.

3.1 Hosting

The basic form of publication on the web is hosting. A server hosts a file if it stores the file on disk or generates the file from data that it stores, and that file did not (to the server's knowledge) originally come from elsewhere on the web.

Accessing hosted pages

A browser makes a request to a hosting server for a file. The hosting server responds with the content of that file, which the browser stores in a local cache and displays to the user.

Diagram showing a browser accessing a document from a hosting server and creating a copy of it within its cache.

The presence of data on a server does not necessarily mean that the organisation that owns and maintains the server has an awareness of that data being present. Many websites are hosted on shared hardware that is owned by a service provider that stores and serves data for other controlling individuals and organisations which determine the data they provide on the site. Because of this, multiple servers may host the same file at different URIs. For example, an artist could upload the same image to multiple servers, which then store the image and serve it to others.

Uploading files to a server

A controller uploads a file to a hosting server, which is then accessed by a browser.

Diagram showing a controller uploading data to a server

There are many different types of service provider. Some may exercise practically no control over the software and data that they host but provide hardware on which code can run. Others may focus on particular types of content, such as images (eg Flickr), videos (eg YouTube) or messages (eg Twitter). There may be many service providers involved in the publication of a particular file on the web: some providing hardware, others providing different kinds of publishing support.

Transforming files on a server

A controller uploads a file to a hosting server, which is then automatically transformed into a different format which is accessed by a browser.

Diagram showing a controller uploading data to a server that is then transformed

Some service providers automatically perform transformations on material that they host, as a service, such as converting to alternative formats, clipping or resizing, or marking up text. When they sign up to a service, controllers explicitly or implicitly enter into an agreement with the service provider that grants them a license to perform transformations on the material which they upload.

Best Practice 1: Transformation of Illegal Material

Legislation that forbade transformations on illegal material would similarly limit the services that service providers could provide.

Service providers that host particular types of material often employ automatic filters to prevent the publication of illegal material, but it is impossible for a service provider to detect and filter out everything that might be illegal. If a service provider automatically transforms that material as part of its service, and illegal material is not successfully filtered out, automatic processing (including the transformation) of files will still take place.

To add to the complexity of this area, it is possible for each of the following to be in different jurisdictions:

3.2 Caching and Distributing

Some servers provide access to files that are hosted elsewhere on the web, on an origin server that holds the original version of the file. These files might be stored on the server and provided again at a later time, in which case for the purposes of this document it is termed a caching server, or might simply pass through the server in response to a request, in which case for the purposes of this document it is termed a distributing server.

Accessing files via a caching server

A browser requests a file from a server; it responds with a document that it has fetched from an origin server.

Diagram showing a caching server passing on to a browser a copy of a document from elsewhere

It is usually impossible to tell whether a server is providing a stored response or has made a new request to an origin server and is serving the results of that request. Servers commonly store the results of some requests and not others, acting as a caching server some of the time and as a distributing server the rest.

In both cases, the file the caching or distributing server provides may be different from the original one that it has accessed from the origin server. For example:

Caching and distributing servers are extremely useful on the web. There are four main types of caching and distributing servers discussed here: proxies, archives, search engines and reusers. The distinctions between them are summarised in the table below.

proxy archive search engine reuser
purpose increase network performance maintain historical record locate relevant information better understand information
refreshing based on HTTP headers never variable based on HTTP headers
retrieval on demand proactive proactive usually on demand
URI use usually uses same URI uses new URI uses new URI uses new URI

3.2.1 Proxies

A proxy is an application that sits in between a server (such as a website) and a client (such as a browser). On the web, caching proxies are often used to speed up users' experience of the web by caching pages and other resources onto the proxy server so that they can be accessed more quickly, directly from the proxy rather than from the origin server. Some proxies, particularly those used by mobile operators, perform other actions on the content that passes through them, such as rewriting or merging Javascript to make it faster to access.

Best Practice 2: Controlling Proxy Behaviour

Proxies should comply with instructions from origin servers that describe whether pages may be copied and transformed, but will only be able to comply with those that are machine-readable.

Proxies come in four general flavors:

  • Forward proxies serve a given community, such as a company. The browsers of the members of that community are set up to use the proxy, and whenever they fetch a resource, the request is routed through the proxy. If the proxy already has a cached copy of the resource, it returns that copy rather than forwarding the request on to the origin server. If many people within the community are requesting the same pages, this can speed up their access and lower the bandwidth use of the community. The proxy can also be used to prevent access to unsuitable sites and to carry out virus checking on the content.
  • Reverse proxies sit in front of a (private) server and cache responses from that server. This can reduce the load on the server and speed up responses, which is particularly important when the response to a request takes time to compute. An extreme version of this is a content-delivery network (CDN) which operates a set of proxies and directs requests to the nearest of these, saving transmission time as well as processing time.
  • Gateways are proxies which usually do not cache the result of a request: they simply pass on the request to an origin server and pass the response back. Gateways may be used to prevent access to particular parts of the web, or to bypass those blocks. For example, a gateway may be set up that prevents access for users in a particular country to particular websites; another gateway might be set up, at a non-blocked location, that enables people within that country to access those websites by mapping requests on that gateway to requests on the blocked website.
  • Transforming proxies are proxies which transform the Web documents being delivered through them to address issues such as presentation issues in order to speed up access to those documents (often these are used to compress content, for example) [reference to Mobile Web Best Practices working group work on best practices for transforming proxies].
Split browsers (such as Opera Mini and Amazon Silk) where some software components execute on a server and some software components execute on a client device are considered out of scope from the perspective of this document. Since the client and the server in these cases are tightly coupled, we consider them to be a single User Agent rather than a client and a proxy.

The use of a forward proxy, gateway or transforming proxy may be configured either on an individual machine or transparently for a particular network. Users may have no idea that their requests are channelled through a given proxy, or they may have configured their set-up to use the proxy.

Reverse proxies appear to be normal servers to users: it is impossible for a user to tell that their request is actually passed on to a completely different origin server, or where that server is. This is intentional as the origin server in this case is a private one.

To improve performance, some proxies, particularly CDNs, may pre-fetch resources that a page includes, since these resources are likely to be requested by the browser soon after the page is viewed. In other words, although generally the contents of a proxy's cache will be determined by the requests that users of that proxy have made, the proxy might also in some cases contain content that no one has ever requested.

3.2.2 Archives

Archives aim to catalog and provide access to some portion of web content to provide an on-going historical record. They use crawlers to fetch pages and other resources from the portion of the web that they cover, and store them on their own servers, along with some metadata about the pages, particularly when they were retrieved. They then provide access to the stored copies of the resources at particular historical dates, enabling people to see how pages used to appear.

Best Practice 3: Controlling Archive Behaviour

Archives should comply with instructions from origin servers that describe whether pages may be copied and transformed, but will only be able to comply with those that are machine-readable.

Archives are usually run by institutions that have a legal mandate and responsibility to keep this historical record, such as a legal deposit. Although their primary purpose is long term record-keeping, they often make this material available online as well. While they might restrict access to the data for a period of time after it is collected, for security or privacy reasons, it is not usually possible to remove information from an archive. Users might use archives for research, but also to access information that has otherwise been removed from the web.

Archived pages are usually distinguishable by end users from the original page using banners placed within the page or having the original page appear within a frame. The links (both to other pages and to embedded resources such as images) are usually rewritten so that when the user interacts with the page, they are taken to the version of the linked resource at the same point in time.

3.2.3 Search Engines

Search engines aim to catalog and provide access to as many web pages as they can, so that they can direct users to appropriate information in response to a search. They use crawlers to fetch pages and other resources from the web, analyse them and store them on their own servers to support further analysis.

Best Practice 4: Controlling Search Engine Behaviour

Search engines should comply with instructions from origin servers that describe whether pages may be copied and transformed, but will only be able to comply with those that are machine-readable.

Search engines are most interested in indexing resources and providing links to them rather than in the content of the resource itself. They might not copy the page itself, but they always store metadata about the page, derived from the information in the page itself and other information on the web, such as what other pages link to it.

Search engines play an important role in the web in enabling people to find information, including that which would otherwise be lost or is temporarily unavailable. When a user views a stored page from a search engine, it is usually obvious both that the search engine is involved (from the URI of the page and from banners or framing), that the content originally came from somewhere else, and where it came from. The links within the page are not usually rewritten.

3.2.4 Reusers

Data reuse is becoming more prevalent as web servers act as services to others. A server that is a reuser fetches information from one or more origin servers and either provides an alternative URI for the same page or adds value to it by reformatting it or combining it with other data. Good examples are the BBC Wildlife Finder, which incorporates information from Wikipedia, Animal Diversity Web and other sources or triplr.org, which converts RDF data from one format to another as a service.

Reusing information

A browser requests a document, which is constructed by the reusing server requesting information from two origin servers

Diagram showing a reuser building a page from information from two origin servers

Best Practice 5: Controlling Reuse

Reusers should comply with instructions from origin servers that describe whether pages may be copied and transformed.

Best Practice 6: Attributing Reused Material

Reusers should indicate the sources of the information on pages, for both humans and computers.

Reusers that do not change the information from the origin server may be used to simplify access to the origin server (by mapping simple URLs to a more complex query) or to provide a route around gateways or the same-origin policy (as servers are not limited in where they access resources from).

Since reused information is, by design, seamlessly integrated into a page that is served from the reuser, people viewing that page will not generally be aware that the information originates from elsewhere. The URIs used for the pages will be those of the reuser, for example. Licenses on the material may require attribution; even when it doesn't, it is good practice for reusers to indicate where the material originates.

3.3 Aliasing

An alias is a URI that points the browser to another URI on an origin server. A server can automatically redirect a browser (using a HTTP 3XX status code and a Location header). Web pages from a server can do the same thing using a <meta> element with an http-equiv attribute set to Refresh; this technique is often used with a slight delay to indicate to the user that they are being redirected to another page.

Aliasing documents

A browser requests a file from an aliasing server; it responds with a redirection to a document hosted on an origin server and doesn't send any content itself

Diagram showing an aliasing server redirecting a browser to a document elsewhere

Aliases do not involve any of the information from the origin server passing or being stored by the redirecting server, but the redirecting server will be able to record when a particular URI is requested.

Although it is preferable to only have one URI for a particular resource, redirections are a useful mechanism for managing change on the web. They are used within websites when the structure of the website changes, or between websites when a new website is created that supersedes the first, or to archived information when a host no longer wants to provide access to a file itself.

Redirections are also used to provide other services. Link shorteners provide a short URI for a resource that is then redirected to the original URI, and are useful in locations where space is limited such as in print or on Twitter. Depending on their implementation, link-tracking services can use a similar technique to enable servers to analyse which links are followed from their site: the link tracker records the request and redirects the user to the true target page.

Best Practice 7: Aliasing Link Shorterners

When aliasing is used to provide a shorter alternative URI to a resource, best practice is to use a domain which is associated with the domain of the destination URI. This allows the end user to know more about where the link will send them and will potentially enhance the longevity of these aliases. Examples are nytim.es and bbc.in.

Add example of how Google records clicks on search results.

When aliasing is used, users may not be aware about the eventual target of a link, or the involvement of an aliasing server, both of which are important. Shortened links, for example, hide the target location behind a URI that often has no visible relationship to the eventual destination of the page. Some implementations of link tracking do not change the original destination of the link (such that the status bar on a browser shows the eventual target of the page) but instead use the onclick event to direct the user to the aliasing server.

Following a redirection, browsers change the address bar to the new location, but this is often the only indication, so users may or may not be aware of this happening.

3.4 Including

Web pages typically rely on many resources other than the HTML in which the page is written, such as images, video, scripts, stylesheets, data and other HTML. The HTML in a web page refers to these external resources in markup, for example, an <img> element uses the src attribute to reference an image which should be shown within the page. Material that is included within a web page may appear to be a hosted copy to the user of a website, but in fact be hosted completely separately, outside the control of the owner of the web page.

Including resources

A browser requests a file from a server; instructions in the page tell the browser to request another resource from a different server

Diagram showing a page including another resource

Best Practice 8: Controlling Inclusion

Includers should comply with computer and human-readable instructions from origin servers that describe whether and how resources may be included within a page.

Best Practice 9: Attributing Inclusion

Those who include third-party resources within their web pages should indicate the sources of the information on such page for both humans and computers.

HTML supports several different ways of including other resources in a page, which are listed in section A. Linking Methods, but they all work in basically the same way. When a user navigates to a web page, the browser typically automatically fetches all the included resources into its local cache and executes them or displays them within the page.

Inclusion is different from hosting, copying or disseminating a file because the information is never stored on, nor passes through, the server that hosts the web page doing the including. As such, although the included resources are an essential component of the page to make it appear and function as a whole, the server of the web page does not have control over their content.

Users may not be aware that included resources are used within a page at all. When included resources are embedded within the page such that they are visible to a user, it won't be clear that an image or video is from a third-party website rather than the website that they are visiting unless this is explicitly indicated within the content of the page.

A resource that is included into a popular page causes a large number of requests to the server on which the file is published, which can be burdensome to the third-party who hosts the file. Publishers who intend their files to be reused in this way therefore typically have terms and conditions that apply to the reuse of those files and may have to put in place technical barriers to prevent it.

As with normal links, included resources may or may not have the same origin as the page that includes them. Resources such as images and scripts that are included within the web page may be from any site. However, browsers implement a same-origin policy which generally means that third-party resources cannot be fetched and processed by scripts running on the page, for example through XMLHttpRequests [XMLHTTPREQUEST] (though typically these scripts can write markup into the page which includes such resources).

3.4.1 Inclusion Chains

When scripts or HTML are included into web pages, the included resource may itself include other resources (which may include still more and so on). The author of the original web page has control over which resources it includes, but will not have control over which resources those included resources go on to include.

Whatever level of remove from the origin page, the publishers of included resources may change the content of those resources at any time, possibly without warning. This has been used in cases where websites included third-party images without permission, to substitute the image with something distasteful or to redirect to a link that performed an action on the user's behalf; see Preventing MySpace Hotlinking.

3.4.2 Hidden Requests

Some of the resources that are included within a page may be invisible to the user. An example is a hidden image that is used for tracking purposes: each time a user navigates to the page, the hidden image is requested; the server uses the information from the request of the image to build a picture of the visitors to the site.

This facility can be used for malicious purposes. An <img> element can point to any URI (not just an image) and causes a GET request on that resource. If a website has been constructed such that GET requests cause an action to be carried out (such as logging out of a website), a page that includes this "image" will cause the action to take place.

3.5 Linking

Linking is a fundamental notion for the web. HTML pages use <a> elements to insert links to other pages on the web, with the href attribute holding the URI for the linked page. Some of the links will be to be pages from the same origin; others will be cross-origin links to pages on third-party's sites that hold related information.

Linking to pages

A browser requests a file from a server; instructions in the page link to other resources, but the browser does not retrieve that resource until told to do so

Diagram showing a page linking to another resource

Best Practice 10: Controlling Linking

Websites that seek to control how other websites link to them should use the technical mechanisms available to them rather than seek to restrict linking through terms and conditions.

A user can usually tell where a link is going to take them prior to selecting (clicking, tapping, etc...) it through the browser UI (e.g. by "mousing over" it) or after the link is selected through the status bar in the browser, although some links are overridden by onclick event handling that takes them to a different location. Some websites, such as Wikipedia, use icons to indicate when a link is a cross-origin link and when it will take a user to a page on the same server. The use of intersticial pages or dialog boxes which warn the user they are about to leave the site in question can obscure the eventual destination of the link, as discussed in section 3.3 Aliasing.

If the link is a cross-origin link (or even in some cases where it is an internal link), the publisher of the origin page will have no control over the content or access policies of the linked page. These are the responsibility of the publisher of that page; the TAG Finding on "Deep Linking" in the World Wide Web [DEEPLINKING] describes the ways in which publishers can control access to their pages and the fundamental principle that addressing (linking to) a page is distinct from accessing it.

3.5.1 Linking Out of Control

Traditionally, a user must take a specific action in order to navigate to the linked page, such as by clicking on the link or selecting it with a keystroke or a voice command. In these cases, the linked page cannot be accessed without the user's knowledge and consent (though they may not know where they will eventually end up).

There are three practices used by some sites that mean that users do not necessarily have control over whether a link is followed:

  1. Browsers can be made to navigate to another page using scripted navigation, which may simply run automatically (navigating the user to another page after a period of time, for example) and can hide the location of a link, such that users don't know where they will be navigating to. The HTML5 history API [HTML5] enables a script to change the address bar, which can mean that the address bar does not reflect the actual location of the page (although this use is limited to locations that have the same origin as the original page).
  2. Browsers might pre-fetch resources that seem likely to be visited next, so that the target page is loaded more quickly when the user accesses it. A page can indicate which links should be pre-fetched using the prefetch link relation in a link. For example, a page might indicate that the first result in a list of search results should be fetched before the user actually navigates the link.
  3. Browsers may support offline web applications [HTML5] which direct browsers to a manifest that lists files that the browser then downloads so that the web application can be used when an internet connection is not available.
It is very early in the implementation of how offline Web applications work. In current implementations, users are not made aware when retrieval of resources from a cache manifest is done on their behalf, however, this may change as implementations mature.

4. Techniques

The description above about how information is published on the web highlights how hard it can be for end users (both human and machine) to be aware of the original source of content on the web, and the ways in which it may have been changed en route to them. It also shows that the controllers of content need to be clear about how that content can be used elsewhere, both through human-readable prose and in the technical barriers that they put up that limit access. Third parties that use that content, whether proxies, reusers or linkers, should also follow some best practices in transformation, reuse and links to information.

4.1 Controllers

Once material is put on the public web (that is, on the internet and unprotected by authentication barriers), it is impossible to completely limit how that material is used through technical means — HTTP headers can be faked, metadata can be ignored. However, there are a number of standard techniques that controllers can use to indicate how they intend their material to be used, which intermediate servers should pay attention to.

4.1.1 Controlling Access

Publishers can control access to resources that are unprotected by authentication through HTTP, by refusing or redirecting connections to particular resources based on:

  • the Referer HTTP header; this is useful for preventing linking to particular resources from outside a website, or preventing the inclusion of a resource in another website
  • the User-Agent HTTP header; this is particularly useful for preventing access from crawlers
  • the domain name or IP address of the client making the connection; this may be useful to prevent specific reusers from accessing material

4.1.2 Controlling Inclusion

As well as the techniques above, which can be used to control any access to pages, it's also possible to provide additional control over the inclusion of resources in a third-party's web pages.

In the case of HTML pages, publishers can include a script that checks whether the document is the top document in the window, to prevent it from being embedded within a frame.

The Cross-Origin Resource Sharing Working Draft [CORS] defines a set of HTTP headers that can be used to give the publisher of the third-party resource greater control over access to their resources. These are usually used to open up cross-origin access to resources that publishers want to be reused, such as JSON or XML data exposed by APIs, by indicating to the browser that the resource can be fetched by a cross-origin script.

A new From-Origin or Embed-Only-From-Origin HTTP header is also currently under discussion by the Web Applications Working Group and described within the Cross-Origin Resource Embedding Restrictions Editor's Draft [CORER]. This would enable publishers to control which origins are able to embed the resources they publish into their pages.

Publishers should ensure actions are not taken on behalf of their users in response to an HTTP GET on a URI, as otherwise sites are open to security breaches through inclusions, as described in section 3.4 Including. It is also good practice to check the Referer header in these cases to prevent actions being taken as the result of the submission of forms within other website's web pages, unless that functionality is desired.

4.1.3 Controlling Caching

There are a number of HTTP headers [HTTP11] that enable content providers to indicate whether a proxy should cache a given page and for how long it should keep the copy. These are described in detail within Section 13: Caching in HTTP. For example, a server can use the HTTP header Cache-Control: no-store to indicate that a particular resource should not be cached by a proxy server.

Publishers of websites can also indicate which pages should not be fetched or indexed by any search engine or archive through robots.txt [ROBOTS] and the robots <meta> element [META]. They can indicate other characteristics of web pages, such as how frequently they might change and their importance on the website, through sitemaps [SITEMAPS]. More sophisticated publishers may use the Automated Content Access Protocol (ACAP) extensions [ACAP] to attempt to indicate access policies.

Publishers can also use the rel="canonical" link relationship to indicate a canonical URI for a page which should be used by search engines and other reusers to reference a given page.

4.1.4 Controlling Processing

The Cache-Control: no-transform HTTP header indicates that a proxy server must not change the original content, nor the headers:

  • Content-Encoding
  • Content-Range
  • Content-Type

For example, an proxy server must not convert a TIFF served with Cache-Control: no-transform into a JPG, nor should it rewrite links within an HTML page.

4.1.5 Link Shorteners

One reason that people linking to websites use misleading links is when the original URLs are too long to incorporate into space-limited documents, such as short-form posts or in printed media. Although it's possible to use third-party link shortening services, origin websites can also set up link shorteners for their own content, and then use the rel="shortlink" link relationship to point from the original page to the short link for that page.

4.1.6 Licensing

Websites indicate a license that describes how the information within the website can be reused by others.

Just as with HTTP headers, robots.txt and sitemaps, there can be no technical guarantees that crawlers will honor license information within a site. However, to give well behaved crawlers a chance of identifying the license under which a page is published, websites should:

  • indicate the license of a web page or other resource using a mechanism whereby the license can be detected automatically such as
    • a <link> or <a> element with rel="license"
    • microformats [MICROFORMATS], RDFa [RDFA-CORE] or microdata [MICRODATA] with a common property such as cc:license to indicate the license of included resources, such as images or videos
  • ensure that the conditions of the license are machine-understandable by
    • referencing a well-known license (such as a Creative Commons license) through its URI, or
    • providing a machine-readable description of the license, for example using the Creative Commons Rights Expression Language.

4.2 Websites

This section describes the techniques that you should use when operating a website that incorporates material from other sources, whether caching, transforming or simply linking.

4.2.1 Honouring Headers

As described in section 4.1.3 Controlling Caching and section 4.1.4 Controlling Processing, there are a number of HTTP headers and other conventions that indicate how an origin server intends other servers to treat the resources that they publish. Servers that cache or transform data from origin servers should obey these headers, which exist to ensure that the end user receives current information in the intended form.

4.2.2 Adding Headers

Proxies must use the Via HTTP header when they handle requests to origin servers, to indicate their involvement in the response to the user's original request. Proxies which perform transformations on a document must include a Warning: 214 Transformation applied HTTP header in the response.

These and other recommendations for proxies which perform transformations are included in the Guidelines for Web Content Transformation Proxies 1.0.

4.2.3 Attribution

Many licenses require the reusers of information to provide attribution to the original source of the material. This attribution must be human-readable, so that users of your website understand where the material came from, and may also be computer-readable, which enables automated tools to track the use of material on the web.

The wording and positioning of attribution is usually dictated by the license under which the material is made available. For example, the license for the free icons available from Axialis Software includes:

If you use the icons in your website, you must add the following link on each page containing the icons (at the bottom of the page for example):

Icons by Axialis Team

The HTML code for this link is:

<a href="http://www.axialis.com/free/icons">Icons</a> by <a href="http://www.axialis.com">Axialis Team</a>

If there is no explicit guidance about the location of attribution, it is recommended to include attribution to material from a third party as close to the use of that material as possible. Methods to make the attribution machine-readable include:

  • the use of the cite attribute on the <blockquote> element, where a portion of a page is quoted within your own site
  • using the dc:source property with microformats, microdata or RDFa to indicate the source of a portion of the page (identified through an id)

An example of clear attribution of material from another site is that of the BBC Wildlife Finder; the following screenshot shows the attribution within the page on the Pygmy Three-toed Sloth.

Attributing Reused Content

A description of the pygmy three-toed sloth is followed by an attribution that reads "This entry is from Wikipedia, the user-contributed encyclopedia. If you find the content in the 'About' section factually incorrect, defamatory or highly offensive you can edit this article at Wikipedia. For more information on our use of Wikipedia please read our FAQ."

4.2.4 Linking

There are a number of practices around linking to a third-party site that can help users and automated agents to understand the relationship between your website and the third parties. These include:

  • use rel="nofollow" for links where the link is not meant to imply approval; these will not be used by search engines when determining the relevance for a page
  • use rel="external" for links to third-party web pages; this can be used as the basis of styling, such as an image that indicates the user will be taken to a separate site

There are a number of techniques that can be used to track which links are followed from a website. Methods that rewrite the links within a web page to point to an interstitial ("you are leaving this website") page or through a script can mislead the user and any automated agents about the target of the link. It is better to use a script to capture onclick or other events and redirect the user at that point.

5. Conclusions

In conclusion, publishing on the Web is different from print publishing. This document has enumerated some of these differences, especially those relevant to licencing and copyright issues.

A. Linking Methods

A.1 Linking by Reference

A.2 Including

B. Acknowledgements

Many thanks to Robin Berjon for ReSpec.js.

C. References

C.1 Normative references

No normative references.

C.2 Informative references

Anne van Kesteren. Cross-Origin Resource Sharing. 17 March 2009. W3C Working Draft. (Work in progress.) URL: http://www.w3.org/TR/2009/WD-cors-20090317
Ian Hickson; David Hyatt. HTML5. 25 May 2011. W3C Working Draft. (Work in progress.) URL: http://www.w3.org/TR/html5
R. Fielding; et al. Hypertext Transfer Protocol - HTTP/1.1. June 1999. Internet RFC 2616. URL: http://www.ietf.org/rfc/rfc2616.txt
Shane McCarron; et al. RDFa Core 1.1: Syntax and processing rules for embedding RDF through attributes. 15 December 2011. W3C Working Draft. URL: http://www.w3.org/TR/2011/WD-rdfa-core-20111215
Anne van Kesteren. The XMLHttpRequest Object. 15 April 2008. W3C Working Draft. (Work in progress.) URL: http://www.w3.org/TR/2008/WD-XMLHttpRequest-20080415