W3C Standards
The W3C is the central standards body for the Internet. As a non-profit organization comprised of industry professionals and academics, they publish a series of guidelines for HTML and CSS, the code from which nearly all of the pages on the Internet are made.
Unfortunately, many of the web pages on the net do not comply with these standards. Standards are needed because there are many different types of operating systems (Windows, Mac OS, Linux) and many different sizes of screens (12", 14", 20", 30", cell phone, etc.), and the same code needs to look good on all of them.
I always attempt to make all of my designs comply with the prevailing standards, XHTML and CSS. Although this is not always possible, since Internet Explorer does not always interpret certain standards correctly, I make design comprises in a way that makes my designs look best on the most common computer configurations.
Not all web designers practice standards-based web design. By clicking on the W3C buttons on my site, you can see that every page on my website validates. I suggest you try validating the website of any web developer you are considering for your project.