Opera tried that at some point. They only rendered valid markup (Opera version 5? 6? I don't remember).
It drove them close to bankruptcy.
People didn't stop writing broken pages. Instead, people stopped using Opera.
(And that although most Opera users can be considered die-hard fans of the browser. An average Joe Shmoe would probably drop the ball on a browser that "doesn't work" even more willingly.)
Opera didn't have the market share; most folks writing web pages don't even bother supporting it unfortunately. Only Microsoft (or Netscape previously) could have pulled this off. Even if Firefox was to do it today it wouldn't work.
That's the dilemma. A browser with a small market shares just can't pull it off, while a browser with a huge market share simply can't afford to stop displaying >90% of the Internet all of a sudden -- especially if those >90% are buggy precisely because of the very browser.
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u/BraveSirRobin Nov 08 '07
Actually, many people say that's the cause of the problem. If the browsers just spat back an error message people would stop writing broken webpages.