For those of us using noise and fractal techniques, it's very easy to create a shader compile which will exceed the timeout in the browser. I think we might need support for async shader compiles. This would enable us to keep the UI responsive with a simple splash screen shader while other shaders are being built in the background. The main thing is to keep the screen from locking up when a long shader compile is needed.
http://fractal.io/ is an excellent example. While my shaders don't take quite as long, I'm hitting the 10-15s duration with shader compiles.
Given that we don't appear to have a simple workaround on the compile side (ie turning off loop unrolling, etc), I think our only option is adding async compiles.