Ire Aderinokun just posted an article on his blog bitsofco.de about linting of HTML with CSS. The idea: visually mark HTML elements with the help of CSS if the elements are broken or incomplete.

A small example: mark all faulty or missing link targets

a:not([href])  
a:[href="#"],  
a:[href=""],  
a[href*="javascript:void(0)"] {
  border: 2px solid red;
}