Gregg Tavares pointed out on the public_webgl mailing list that not supporting the "shared" and "packed" uniform block layout qualifiers will cause many OpenGL ES 3.0 shaders to not work in WebGL 2.0. This issue was filed here:

https://github.com/KhronosGroup/WebGL/issues/1534

This should be resolved before WebGL 2.0 ships.

ANGLE bug https://bugs.chromium.org/p/angleproject/issues/detail?id=1542 was filed about implementing this semantic in the shader translator. This flag should be enabled in Chromium once available, and conformance tests should be written that guarantee that these uniform block layout qualifiers work in WebGL 2.0.