Well in many browsers for example there is a internal warning and error log (window). Which yet does not (need to) claim to be a official W3C checker. It has positive effect on web stabilization. 
For example just looking now I see the many HTML and CSS warnings and errors about the sourceforge site and this bug tracker in the Browsers log - not believing that the log covers the bugs 100% ;-)

The events of warnings are easily available here, and calling self.warning, as it was, costs quite nothing. I don't see a problem for non-users of this feature. And most code using HTMLParser also emits warnings on the next higher syntax level, so to not have a black box...
 
As I used a tolerant version of HTMLParser for about a decade, I can say the warnings are of the same value in many apps and use case, as to be able to have look into a Browsers syntax log. 
The style of stretching a argument to black<->white is not reasonable here in the world of human edited HTML ;-)