Quote from debian bug (http://bugs.debian.org/630943:
I have noticed several minor issues with the HTML output of awstats
1) it mis-marks its xhtml output as conforming to xhtml 1.0 strict, but it
is really transitional (as the non-xml html output is marked)
2) the non-xml html declaration doesn't include the system identifier.
this causes tidy to not recognize it, and also means that gecko browsers
use quirks mode rather than standards compliance mode.