If you are a web site developer, you have heard of DOCTYPEs, and HTML or XHTML. Or, maybe you use some code-generating WYSIWYG1 tool, and know nothing about them. If this is the case, you should learn.
I will offer three test pages for you to look at, especially with Internet Explorer. Note how that inferior browser handles properly-served XHTML. (Press your back button to return here.)
Rather than rehash all the "whys" and "why nots", I'll give you two links to good pages that explain why you don't want to use XHTML. Thanks to these two authors for taking the time to write these pages.
Now are you convinced to not use XHTML?
1. WYSIWYG: What You See Is Maybe What You Get If You Are Lucky. These tools include DreamWeaver (for folks with lots of money), Microsoft FrontPlague (oops, FrontPage) or Microsoft Publisher (ewww!) for those on a budget, and many others available for free or for a minimal charge.