In a comment on my previous blog post, Simon Willison writes: "I'm not too keen on popping up a new window with an external validation service. How about an alternative bookmarklet that just validates XML well-formedness (essential for those of us who use XHTML)?"
Try the new blogidate well-formedness bookmarklet. If this bookmarklet finds an error, it turns the textarea red, selects the part of the textarea where the error is, and puts the error message in the status bar (not in a dialog). If it doesn't find an error, it turns the textarea green.
Update Jan 21, 2005: added a normalize() call so the bookmarklet won't fail when the XML is more than a few kilobytes.
Update Sept 2, 2005: the Valid XHTML user script, which is based on this bookmarklet, is even more cool.