The WebGL extension OES_texture_half_float_linear is unavailable on iOS (tested locally on the simulator) with WebKit's ANGLE backend for WebGL.
ANGLE isn't advertising this extension to OpenGL ES 2.0 contexts when it's using an OpenGL ES 3.0 context under the hood.
Once Bug 210524 is addressed and OES_texture_half_float is properly exposed, this must be fixed as well to avoid regressions.
Created attachment 399915 [details]
Attached is the needed patch to ANGLE. Not ready for review or EWS testing yet; it depends on the patch for Bug 210524 landing.
Have tested this minimally on the iOS Simulator; it makes the oes-texture-half-float-linear.html test pass, and the extension is actually enabled.
Created attachment 399918 [details]
Note that there are important steps to take when updating ANGLE. See http://trac.webkit.org/wiki/UpdatingANGLE
Committed r261976: <https://trac.webkit.org/changeset/261976>