I once used Zoom's web interface and the video quality was horrible. When I switched to the downloaded app, it was much better. Not sure if this was a one-off problem or an underinvestment in their web interface.
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They also limit the resolution to 480p for the web app, probably because of performances. Browsers and zoom both use h264, but browsers usually use vp8 instead.
There is no reason webrtc cannot offer the same quality (or better) than zoom, at the same bit rate, but it all comes down to the actual implementation of browsers and web apps. webrtc-based apps work well, these days.