Many people underestimate the importance of valid markup. There are rules for proper HTML and XHTML, and there are reasons for those rules. WYSIWYG layout editors made many web designers (including myself) lazy. We were rightfully worried about what the page looked like, but often neglected the importance of what the code looked like. In fact, Internet Explorer shares much of the blame as oftentimes web developers had to purposefully write invalid markup as Internet Explorer didn’t (and in some cases still doesn’t) render proper code properly. But when you neglect your markup, eventually your web page itself will fail.