Currently the WebKit implementation of WebGL keeps around the current index array so it can be used to do bounds checking on draw calls. This is mandated in section 6.4 of the spec. The ARB_robustness extension requires that out-of-bounds array accesses be forbidden. But by my reading it doesn't require the behavior mandated in the WebGL spec. They simply guarantee that no fetches outside the array will happen. Compliant behavior would be to simply return 0 values for these accesses. So bounds checking on the WebGL side would still be required.
If this is the case, I think we should make changes so drivers implementing this extension can avoid the WebGL side bounds checking. I'm not sure it's practical to change the ARB_isolation extension at this point. So I would be in favor of changing section 6.4 of the spec to match the behavior required in the extension.