Like transclusion, annotation is an interesting hypertext technology that has not been widely implemented on the web. However, unlike transclusion, annotation fits well into the web architecture and could easily be added with minor extensions to existing browsers.
Annotation involves attaching external content to another document by reference instead of by modifying that document. It can only be approximated in current web browsers using complex JavaScript, which is difficult to do in a cross-platform manner.
Since annotation is not available, a haphazard combination of links and quoting must be used instead. This reduces the potential for collaboration by preventing discussion from taking place right in the document being discussed. There is no excuse for the lack of annotation on the web and several good reasons for adding it.
Annotations are efficient
Both annotations and transclusions require multiple HTTP requests to assemble a page by retrieving the annotations or the transcluded content, respectively. However, annotations incur much less noticeable latency for the reader.
Annotations are retrieved from a set of annotation providers that generally are chosen before viewing pages. This allows annotations to be indexed and retrieved in advance, a form of resource precaching. This optimisation is not available for transclusions, as the sites from which content is to be transcluded are not known until the page is loaded, and cannot be precached.
The effect of latency on annotations is still minimal even if the annotation providers are chosen dynamically after the page has been loaded. Since the page will still be readable and coherent without annotations, the reader will not be greatly inconvenienced if they take several seconds to appear. However, latency while retrieving transclusions may lead to incomprehensible pages until all of the transcluded content has been loaded.
Annotations degrade gracefully
Web links break; this is an axiom of the web architecture. Web links used for annotation will also break, but the consequence of a broken annotation is far less irritating than a broken transclusion: the annotation simply will not appear, or will appear in a default location indicating that the link has been broken. Annotations degrade gracefully in the presence of broken links without impacting other content, while broken transclusions can render entire pages meaningless.
Annotations are safe
Annotating another web site is a perfectly legitimate and safe activity, as annotations cannot be hijacked by malicious operators in the way that transclusions can. Annotations must be explicitly requested by the reader and cannot change the content of other web sites, only annotate it. (Although this does indicate the need for a clear visual distinction between annotations and the underlying web site, to avoid confusion).
It is possible that the operator of a web site that has been annotated by someone else might modify the content of their site to make those annotations look foolish or outdated, but that is a problem that affects any method of quoting or commentary, including transclusion.
Annotations are expressive
Annotations can link to arbitrary locations in other documents and may contain arbitrary markup themselves, including structural or semantic constructs and also style information. There is no architectural limitation on the expressiveness of annotations as there is with transclusion.
Annotations belong on the web
Imagine an online edition of a classic novel, annotated for schoolchildren as is commonly done with printed editions. Spelling variations or archaic words could be annotated with their modern equivalents and major themes could be cross-referenced or linked to in-depth dicussions. Even better, why not ask the students to annotate sections of a book themselves as they study it? Annotation could also be used for marking and correcting work online, as a high tech alternative to the dreaded red pen wielded by teachers around the globe.
There have been many prototypes of annotation for the web, Annotea being a recent example. None of them seem to have become popular, which is strange considering the popularity of other new hypertext technology such as RSS and Atom syndication.
Syndication formats could play an interesting role in annotation on the web: you could subscribe to an annotation feed that provided annotations from other readers. This would be the equivalent of a linkblog, but each entry would sit silently in your browser until you stumbled onto a site where another reader had been before, to see that ‘Kilroy was here’.
Copyright © 2004 Michael Day, mikeday@yeslogic.com