The XHTML 1.0 standard has a non-normative appendix of Compatibility Guidelines – ways to code your XHTML to make it render reasonably well in HTML user agents. Of course, being the browser-neutral standards body that they are, the document doesn’t say exactly which browsers require which compatibility workarounds.
It would be rather cool if someone were to knock up a test page which used each of the possibly-problematic constructs (e.g. <br/> without the space, checked=”checked”, line breaks in attributes) and then made a matrix from the results people sent in when they pointed different browsers at it. This would give web developers some idea of which of the requirements were only for the benefit of long-dead browsers like, er, Netscape 4.
Any volunteers? :-)