Where HTML Browsers Aren't Real Smart

A single vocabulary HTML's biggest problem isn't simply structure vs. formatting - it's that HTML itself can't possibly do everything that developers want to see happen in a browser interface.
Hypertext HTML's simplistic approach to hypertext helped it take off, but its limitations have grown clearer and clearer as hypertext has grown.
Plain Text-based inputs HTML forms let you work with a limited variety of input types, all of which are reported as simple text.