There are two kinds of backwards compatibility: the obvious one of a version of a specification with previous versions of the same, and another one of new technologies with earlier ones.
Nobody forgets about the former, because there is nothing the developers of a new version know so well as the previous version they are trying to replace. Backwards compatibility is always hotly discussed.
But the latter is less obvious. It is, in a sense, the complement of extensibility and modularity. Whereas those two stress the importance of developing technology in such a way that it will work together with future new technologies, backwards compatibility stresses the importance of working well with what is already there. No new technology is designed in a void.
Not only does a new technology normally have to be compatible with earlier ones in technical matters, but also in the mental models that users have of the old technology. Introducing new paradigms always has a cost, that has to be set off against future benefits.
Take the example of CSS: people using HTML, and indeed most word-processors, are used to attaching stylistic information to concrete elements. To make style sheets acceptable at all, CSS has to allow people to continue working in the same way, while expanding their options.
As a counter-example, XSL takes a different approach to styling: people first create the style in the form of a template and then inject elements into it. It requires more motivation from users, but therefore it offers new possibilities. It aims at users that can afford to learn about recursion and loops and that need to style abstract documents that will be generated in the future (dynamic reports from a database, for example), rather than concrete articles that have already been written.
Sometimes a new technology replaces a previous one, although it is rare that it replaces the old one completely. PNG in principle is able to replace GIF completely, and XHTML can completely replace HTML. But even in such cases, some form of backward-compatibility is needed in the form of facilties for mechanical conversion of resources to the new format.
“Mechanical” means that the amount of intelligence necessary should be limited. For example, SVG partly replaces PNG, viz., in cases where PNG is used to display diagrams or graphs. Those would be better in SVG, but the backwards-compatibility of SVG with PNG is almost non-existent. Of course, in this case the benefits of SVG for images of this kind is so great that nobody complains about the lack of compatibility (but even so, SVG provides a trivial way to do the conversion: wrap the PNG in an SVG object without any interpretation).
The Web itself is designed to be backwards compatible. The URLs that give the location of resources allow for access via FTP and other protocols rather than just HTTP, and HTTP in turn allows for arbitrary types of files, not just HTML. And, as a final example, HTML allows any kind of file to be hyperlinked (via the A element) or included (by means of the IMG or OBJECT elements), not just HTML and PNG.