Looking at a page on test I got this error msg:
"There were serious xhtml markup errors detected by
tidy."
Page had not changed in days and was always OK.
After minor edit (adding a space) page was shown
properly rendered both on preview and after save.
Returning some time later the same error had
reappeared. See
http://test.wikipedia.org/wiki/EasyTimeline/Testing
Erik Zachte
Logged In: YES
user_id=1039118
see meta bug page
5 users reported same error on different pages
Logged In: YES
user_id=956757
Tidy wasn't installed on the new servers which displayed as
a grave error. Reporting is disabled now as well, only a
comment is added if tidy gave up on the markup. Didn't come
across a page where this was the case after tidy was
installed on tuesday.