Dave Hyatt works for Apple on the renderer WebCore that Apple uses for Safari (and shares with Konquerer). He has a blog in which he discusses various interesting aspects of his work:
In particular, as I recall, he has expressed the idea that the hard part isn't handling standard compliant markup, but gracefully handling uncompliant markup. Here's a relevant post from him (in the context of discussing how XML error failure is different from HTML error failure):
Fully half of the bugs I receive in WebCore are not bugs at all, but are essentially differences in error handling and error recovery between Safari and the dominant Web browser, WinIE. None of these issues occur with XML.
If we lived in a world where browsers could refuse to display malformed content (with useful error notification of course so that authors could easily repair their content), then all of these "bugs" would simply disappear. I could focus my efforts on real DOM and CSS bugs, and not have to waste my time emulating the behavior of WinIE.
So, as previous posters have said, if you have clean markup, you know what to expect and can reasonably be unhappy at browsers that don't comply. If you have erronious markup, then you have to just hope that every browser deals with the error in the same way the Author's browser dealt with it.