One of the capabilities inherent in the specification's foundation is the ability for an author to associate more than one stylesheet with a document, thereby allowing the user to choose which they prefer.
For example, this document has been linked to three stylesheets. The first, default, set of styles suggests a blue scheme. The second set of styles give the document a different, green, appearence, and the third is specifically tuned for users who are visually impaired (in this case displaying the text using more contrasting colors).
Adding this capability would greatly enhance not only the author's ability to assist users, but also improve the accessibility of his documents.
No browser currently boasts a feature whereby when viewing this page the user can ask to be presented with a method of selecting which of these three alternate stylesheets to use. This is an area where Opera Software has a chance to really set itself apart from other browser vendors.
If you wish to see what the feature could achieve, then try following each of the following links. Each page contains an exact copy of this document, but is explicitly styled using one of the three sheets described above.