What an interesting question! Who knows, but here is a thought...
I think XHTML and CSS will be more popular in the future because it is a universal file format for structuring semantic content (XHTML) and styling and laying out out that content (CSS).
Because XHTML enhances the meaning of content (through structural and semantic tags) and because CSS separates style and layout from the content, the content is more meaningful to search engines and screen readers. Also this approach degrades nicely on internet-connected devices such as cell phones and PDAs that have small displays.
I personally think the Internet is ultimately about content and interactivity is best used to augment content.
On the other hand, I think it is a different story for interactive applications (games, mashups, etc). A lot of different technologies will compete for developer mindshare for a internet-delivered applications...Personally I think .NET 3.0 will win that race in about 5 years...not that I think this is a good thing :-)
Just some thoughts...I don't have any strong opinions about this...
subject: "Pro CSS and HTML Design Patterns" question: will JavaFX be more popular than HTML?