There are accually lots of things like how fast the site is loaded. eg. It will load much faster if you use .css. Also, the website has to look the same in every browser.
also some supa noobs have it so if you well load the page and its minimized the navagation menu or something will collaps and look really crappy. also you can tell if anything is way out of proportion like a hugh space between the end of the content and the bottom of the page
In good coding, Validity is of one of the top priorities. Poorly coded pages "optimized" for only one browser is not recommended. Coders should not abuse the use of Javascript, and should keep things simple. Coding is just as important, if not more, as design. You can also view the source of the pages by right-clicking the page.