Ahh! I was misled by the fairly large number of extensions with 0% support. They are actually 0% after rounding.
According to https://www.semiwiki.com/forum/content/3169-astc-new-midrange-arm-mali-t720-gpu.html ASTC is supported by the ARM Mali-T720, Tegra K1, PowerVR Series6XT and Adreno 420. The source of the information is not described. All these companies have announced that they will support ASTC and, as a cross-platform solution, it is very likely to quickly build momentum. Therefore it seems a good idea to prepare a WebGL extension spec. now rather than waiting until ASTC is already in a large number of end-user devices.
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