In the past, many authors who relied on the quirks of Netscape 1.1 suddenly found their pages appeared totally blank in Netscape 2.0.
Whilst Internet Explorer initially set out to be bug-compatible with Netscape, it too has moved towards standards compliance in later releases.
As of today, there is little or no certification for Web professionals, and only few universities teach Web technologies, leaving most Web-smiths to learn by themselves, with varied success.
Seasoned, able professionals will take pride in creating Web content using semantic and well-formed markup, separation of style and content, etc.
Validation can then be used as a quick check to determine whether the code is the clean work of a seasoned HTML author, or quickly hacked-together tag soup.
Some teachers also stress that automated validation tests are a good introduction to broader, more complex quality concepts such as accessibility.
Creating Web pages or applications according to a widely accepted coding style makes them easier to maintain, even if the maintenance and evolution is performed by someone else.
Many professionals have been authoring the Web with HTML and CSS for years and know these technologies by heart.
This is deliberate, and doesn't imply any kind of browser bug.
A term sometimes used for this is WYSINWOG - What You See Is Not What Others Get (unless by coincidence).