This happens all the time; Firefox adds non-standard CSS transforms, Microsoft adds non-standard Javascript functionality, and Apple adds proprietary video codecs to Safari. When your web app depends on these things, you hurt your users. But it's your fault for using nonstandard features; it's not Apple's or Google's or Microsoft's fault for making their browser support them. If this upsets you, consider software in general. To use Facebook, you have to use Facebook. That's nonstandard! To use an Emacs extension, you have to use Emacs. That's nonstandard! And so on.
Google and Mozilla are especially innocent, since Apple and Microsoft can easily steal any features they want from Chrome and Firefox. They're the only ones out in the open, and for that, I think they deserve to try new things for the web. If they stick, the other vendors can easily support the new features.
then "Google and Mozilla are especially innocent, since Apple and Microsoft can easily steal any features they want from Chrome and Firefox. They're the only ones out in the open, and for that, I think they deserve to try new things for the web. If they stick, the other vendors can easily support the new features."
How would these features stick, if developers using them are to blame?
You talk as if blame is a conserved quantity. Just because it's the developer's fault for doing something, doesn't necessarily mean that it's not Google's fault as well for enabling them to do so.
When Google makes these technologies, they are aware of how they will, or can, be used, and that it may involve some fragmentation of the web. Aware of these consequences, they went ahead and released the technologies anyways. In my mind, that entails them a degree of responsibility for the consequences.
It's also difficult for me to take them at their word when they say "we think this problems an acceptable cost to move the web forward," because they also happen to benefit from this fragmentation. I don't think they're being intentionally duplicitous, I just don't think it's possible for anybody to make fully rational decisions in such a situation.
You realize that if not for Flash, sites like Youtube wouldn't have been possible at the time that it was released?
[1] ActieX didn't pan out for the web as a whole - it fared well behind the walls of enterprises.
Standards bodies move slowly. Non-standardised innovation across browsers is absolutely to be encouraged, as long as it is done in a manner that allows others to implement that innovation too. This sort of innovation is what helps us decide what is worth being made into a standard.