The WebGL problem is from webkit adding support for OffscreenCanvas but with missing webgl support. The workaround, until proper support, is to not only check if OffscreenCanvas is available, but if it has the webgl context too.
The missing transparency in hevc videos is with no workaround as far as I know. I also don't know if it has anything to do with the missing webgl support. I think it should be a separate issue in the forum...
stevestavropoulos
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