BOINC website have "poor" quality code.
For example: (from index.php)
<hr width=80% size=1>
In new web standards it should look like:
<hr style="width: 80%; height: 1px" />
The code need some improvements. If not XHTML improvements, it needs quotes (" ") in tags values.
Additionaly I suggest to use CSS files. Save them outside the php file and link to elements by "id" or "class" tags.
I think that the XHTML standard of website is very important and it is the future of Internet.
P.S.
I offer my help with working on this task. I made some XHTML 1.1 valid sites and I can spend some time (approximately 8h-10h per week) to make the code valid.
Edit by Nicolas: XHTML won't be used, we're staying with HTML4.01; see comment 4.