Actually it is better for security.

Proof please?

Whenever I visit a WebGL demo inside Linux w/ NVidia drivers, if there are any complicated shaders, my system hard-freezes, forcing me to reboot.

Until the X developers fix Xorg's security model (like that will happen), and until NVidia fixes their Linux drivers (like that will happen), WebGL will always be one massive gaping security and stability problem for me as a Linux user.

So, you are saying the same thing that I am :D

Because it runs userspace code on x86, instead of running kernel-level code on the GPU?

Because we know, those days, to sandbox raw x86 code, and not let them access the system and the drivers... However, running shaders directly on the GPU exposes directly graphic drivers, that are not conceived to block unfriendly code.