Programmatic cross browser testing?

I've recently moved from backend programming to webapps. Obviously cross browser testing is an annoying problem, and I'm wondering if there is a tool to help that.

There are tools like Selenium/Webdriver that push buttons, but I'm looking for something that parses a website's HTML/CSS/JS and warns you about problems. Sort of like "you're using an X tag inside of a Y. That is known to look poor on IE6". I guess you could call it lint for the web.

Does such a thing exist? If not, why? It sounds useful to me.