Seems like about half the issues you've mentioned relate to differences in syntax/behaviour.

If you mean the bugs I described, then no, I think all of them are genuine bugs.

There's no way to justify Chrome not redoing page layout or repainting properly after a DOM or CSS update. That's just sloppy, and it's a serious problem if you're writing app-style pages. Unfortunately, I don't have a self-contained, reproducible test case: we see this a lot using toolkits like jQuery UI when showing (or not) dialog boxes, but unfortunately it's a bit of a Heisenbug, or possibly several Heisenbugs. The general issue has been reported somewhat widely in Chrome-related blogs, though, and IIRC there were some issues already in their bug tracker that looked like they might be related. The latter led to some blog posts suggesting workarounds with JavaScript that might correct the CSS calculation errors in at least some of the cases. The issue has been there for several months, and still hits us from time to time.

The rounded corners problem I referred to was a rendering problem that made pages using the CSS3 rounded corners horribly ugly and which seemed to take several attempts to finally get right. (Of course there were browser prefixes and so on in this area as well for a time, but that's not what I was referring to.)

The H.264 issue is just a mess. The fact is that Chrome supported it, and then Google said after a future update they wouldn't support it any more. They were planning to push an update that outright removed functionality, and that functionality was in one of the few cutting edge areas that was really taking off at the time, too.

And Firefox screwing up support for the plug-ins was just poor quality checking before a release. There's no excuse for breaking something on that kind of scale, particularly when the bug was already in the tracker several days before the update was pushed out.

Sure they're sometimes broken. Sure we're all effectively beta testers. But everyone knows that.

No, they really don't. When my clients ask why the software they've paid me a lot of money to write for them has stopped working, with the result that their on-call guys are getting woken up in the middle of the night by someone on the far side of the world who can't use their very expensive equipment any more, the answer they are looking for is not "Well, you see, we use a Java applet in this product because when we started the project a few years ago HTML5 canvas didn't have the functionality, speed or portability we needed, and that Java applet requires that the user have a Java runtime installed on their system and a corresponding plug-in for their browser, and your customers are using Firefox as their browser, and Firefox pushes automatic updates, which in a stunning piece of incompetence your customers' system administrators have not disabled, so their key systems will apply the updates before anyone tests that nothing important breaks, and in one of those updates the Firefox developers kinda broke a bunch of things on pages that use applets, and that's why the users can't type text where they should be able to, and so it's really not our fault, but at least these bugs will probably get fixed in the next release and that probably won't be more than six weeks away, so for now your platinum support guys should tell your customers to just hang in there for some unknown length of time and then they'll be able to use that very expensive hardware again, and in any case I'm sure this won't in any way jeopardize that contract you were going to get to sell them another 250 units in a multi-million dollar deal next month." No, trust me, that's really not what paying clients want to hear at all.