Almost done
by Volker Weber
Haiko was right. The Validable plugin did the trick. However MT does not behave nicely with block constructs that span paragraphs. It recognizes the opening <blockquote> tag but not the closing </blockquote>. So it encloses that in paragraph tags and thus the code is no longer wellformed XML. Bummer.
There is a workaround: Avoid empty lines in block constructs and put two <br /> tags there, joining all lines.
Instead of doing this:
<blockquote>text text text text text text text text text
text text text text text text </blockquote>
You have to do that:
<blockquote>text text text text text text text text text<br /><br />text text text text text text </blockquote>
As I am not going to fix all previous posts, this site will only validate from now on.