In life, there are a thing called "standards". Standards are a way to set rules for how, when, or why something should be used. This is done so that different things made by different people will work the same way. For instance, the width of a railroad is a standard; it ensures that any train from any company can work on all tracks.
These kinds of things are useful to everyone. Imagine if every car used a different type of gasoline. Gas stations would be impossible! Well, the Internet also has standards when it comes to things like designing a website. These stanrdards are put in place so that users can choose any browser they like and view a webpage in the same way, regardless of their choice.
Unfortunatly, this doesn't work too well on the Internet. Microsoft, the software monopoly, has a browser. I'm sure you've heard of it. In fact, chances are that you're using it to view this very page. Internet Explorer is used by 80% of all web surfers, but it horribly detracts from standards set forth by the World Wide Web Consortium (W3C), which is in charge of such standards.
Because of the popularity of IE, many sites are tooled to fit Microsoft's inane little quirks when it comes to such things as HTML and CSS rendering. These changes often break the page on standards-compliant browsers, such as FireFox (which I highly recommend as an alternative to IE.) Every web designer should follow standards, regardless of what Microsoft does.
To this end, I design all my websites to be standards compliant. Now, the code can seem a bit off (for those of you who can understand that,) and that's because I've made concessions to make it work right in IE; after all, you can't just ignore 80% of the market. But my changes are always standards compliant, as it should be.
This site validates because we support web standards.