I have working on making all my pages as close to clean XHMTL as I can. One of the tools I use is HTMLtidy and I have it rewrite my pages as it goes through my site.
cd /www/movabletype; find . -name "*.html" | xargs /usr/local/bin/tidy
That leaves the pages machine-verifiably valid, but not what I’d call good-looking. One issue was in the individual entry pages: the navigation menu to go to next entry, previous entry or back to the main page was broken across three lines. Tidy was wrapping a <p> container around each element. As it turns out, it was taking its cue from the markup in the template.
Condensing it all down to one line:
<div id="menu">
<MTEntryPrevious><a href="<$MTEntryPermalink$>">« <$MTEntryTitle$></a> | <
/MTEntryPrevious> <a href="<$MTBlogURL$>">Main</a> <MTEntryNext> | <a href="<$MT
EntryPermalink$>"><$MTEntryTitle$> »</a> </MTEntryNext>
</div>
fixed that.
Same with the footer elements (posted by, comments, trackbacks).
With those simple changes, 1500 pages now pass muster as valid XHTML 1.0.
Now to see what’s up with the main index page: 84 errors there, and a few ugly reformattings/rewrites to fix.