No one gives you unsecured access to the graphics stack. WebGL shaders are parsed and modified for security (for example, inserting things like bounds checks).
(Of course, there are security risks with any new technology.)
I have similar thoughts about allowing web sites to install fonts. Font rendering is hard, and font engines have not traditionally been exposed to attack. The idea that we have found all the bugs in them is absurd. There were security problems found in png many years after release, and that was something people knew needed to be secure from the beginning.
That could be an argument against new JavaScript JITs, too, or new video codecs. But we add those to browsers all the time, because they are important. So is being able to render 3D, I would argue.
Furthermore, of course we have an idea of the risks here, and the ways to prevent them. A huge amount of effort has gone into that, but in speccing WebGL and in implementing it, and in collaboration with GL driver vendors. And security experts have been poking at GL and WebGL for a while, just like they poke at JS engines and video codecs.
3D is also a lot to give up.
I understand if you happen to not care about speed or 3D, and that's fine, but most people do.
In particular, I think allowing malicious people to run programs on your computer by default because, hey it's safe in the sandbox, is a terrible idea.
Fair enough, I agree but put the line in a different place.
Another example: When you get bufferData in WebGL, it doesn't just call glBufferData with those arguments. It carefully checks those arguments for validity, including against the current state (currently bound buffer, its size, etc.). This ensures that the GL driver only sees calls with valid, reasonable info. It does incur a performance hit, but as you can see from WebGL demos on the web, it isn't terrible.
GL drivers in general are pretty complex. But the very small subset of GL that WebGL uses - both in terms of the commands used, and the content it passes in those commands as just mentioned - is much simpler to secure. Browser vendors have been working with GL driver makers on safety for a while now to improve things further.
Again, are there security risks? Sure, any new technology has them. JavaScript engines and video codecs have them, exploits for those are discovered all the time, but without those our browsers would be much less useful. The same is true for WebGL.