All modern browsers have done GPU compositing for a while now, so if anything GPU decode to a texture is the nicer path anyway.

As for HTML5 video allowing JS to get individual pixels: no one made any performance guarantees. The expectation is if you use these APIs (don't!) that you will incur at minimum a full framebuffer copy into a CPU shadow buffer, and likely hog your AVX-esque units that have to decode from some GPU-specific format into raster or do colorspace stuff.

Note that in this thread we equate hardware video decoding with GPU, but on mobile or mobile-derived hardware this is not at all the case. Your hardware video decoder is likely separate silicon that prefers to decode into its own YUV-happy format, and that format is in turn more or less only guaranteed to be understood by the hardware compositor - the GPU might not be able to share it. So if you start trying to do anything with video beyond displaying it in it's own plane, you can very easily force the hardware into a path that will be slow and energy consuming. This is well understood by the people that build Android and iOS, not so much web developers.