The news about Opera switching to Webkit is disappointing. It's a sad day for the Web, since I thought highly of their Presto engine and their Web standards work. Their impact on Web standards will be dramatically reduced, especially where they want to do something differently to Apple and Google. The big loss for the Web is a further decrease in the diversity of browser engines, especially on mobile devices. This means that our work at Mozilla simultaneously becomes more important --- we now have one of the three viable browser engines, instead of one of the four, and engine diversity is already critically endangered --- and more difficult, because this will strengthen the Webkit mobile monoculture and make it even harder for us to promote Web standards over "coding to Webkit".
Some people are wondering whether engine diversity really matters. "Webkit is open source so if everyone worked together on it and shipped it, would that be so bad?" Yes. Web standards would lose all significance and standards processes would be superceded by Webkit project decisions and politics. Webkit bugs would become the standard: there would be no way for developers to test on multiple engines to determine whether an unexpected behavior is a bug or intended. It wouldn't be the worst possible outcome --- victory of a closed-source engine would be worse --- but it would be a far cry from the open Web goals we've been striving for.
So, back to work!