Following up on an old article form Jesse Ruderman I wanted to have a way from within WordPress to validate XHTML through the Quicktags buttons. I rewrote the "Bloggidate" set of hacks to work from within WordPress.
So this will let you validate the content of your post before you actually post it to the blog to ensure things do not ever break in terms of validation. Just type in the post and click on validate from the quicktags.
Get more information here: