What's stopping me from using arbitrary tags in HTML?

Question

Even the new HTML5 tags aren\'t enough to describe structures without falling back to divs. What\'s stopping me from changing:

Answer 1

One reason is that Internet Explorer (and earlier versions of Firefox) don't have a fallback for tags that are not defined and wind up ignoring them for both styling and nesting. Without a shiv, your site will break horribly in those browsers.