Sites that use CSS with either XHTML or HTML are easier to tweak, so that they appear similar in different browsers.

Sites using CSS ‘degrade gracefully’ in browsers unable to display graphical content. Browsers ignore CSS that they don’t ‘understand’, such as CSS 3 statements. This enables a wide variety of user agents to access the content of a site even if they can’t render the stylesheet or aren’t designed with graphical capability in mind.

For example, a browser using a refreshable braille display for output could disregard layout information entirely, and the user would still have access to all page content.

Source: Wikipedia