You could probably match some patterns, but things can get really complicated when you introduce javascript into the mix. All of a sudden your tool needs to test every button on a page to make sure your ajaxed data tables won't blow up your layout.

Also, web development tends to push the envelope often. I've built literally hundreds of web sites / apps and I still come across nasty unexplicable post-ajax redraw bugs in IE6 that I've never seen before (using markup that I used to consider "safe", but which turns out to break under a new novel condition).