I just pasted some generated javadoc into an eclipse project, to discover none of the HTML is compliant.
There is even cases of closing tags that were never opened.
The short answer is that JavaDocs predate XHTML.
There was an interesting thread on the Java Posse group about this recently that you might find interesting: why is javadoc stuck in 1997?
After some googling, I discovered XHTML Doclet 0.4.
XHTML Doclet is a standards-compliant alternative to the Javadoc standard HTML doclet. The project revises the document structure to exclude outdated tags and inline styles, creates valid XHTML markup, and provides better hooks for more flexible CSS manipulation.
Looks like someone made a plugin.
Any better options?
Edit: Here's the plugin's Official page as linked to by Sun's Javadoc FAQ.