You can't completely rewrite various subsystems for performance and fix bugs at the same time. There is more going on than just 'adding new features'. New software is bound to be unstable while it is in development. If you have a problem with this, do not download beta versions.
Personally, I haven't had any issues at all with stability or compatibility on Beta 6 or 7 (on Linux).
And here's my snarky rejoinder: Hey look! There are bug fixes and new features in the latest beta. How did they do that? Perhaps they misunderstand the software development process!
What you have described is some ideal model of the development process based on something you think should happen. It doesn't exist, and it doesn't happen that way.
No one is reading what I wrote: their priority is messed up (in my opinion). Software development is very much about priorities.
You can decide that youtube functionality might be important for mac users for them to even continue testing the beta (e.g. no they(I) aren't(am not) going to keep launching another browser for every you tube video when they could just use safari). Or you can decide WebGL and audio (audio? really?) bling which a precious few will be using even in a year, is somehow more important.
As far as the final product is concerned, I'd bet that YouTube does have higher priority than WebGL. But that isn't the current focus of development.
> What you have described is some ideal model of the development process based on something you think should happen. It doesn't exist, and it doesn't happen that way.
Do you actually do any programming yourself? I'm really not sure what you're talking about.