So, the JavaScript audio/video APIs already available in the browser are fully competent to build any kind of media player. The major limitation is that media players reliant on the browser APIs are limited to the codecs and containers supported by the browsers, which turns out to be a huge limitation.
A media library written for WASM with supplied codecs would not have that limitation, such that you could have an equivalent to VLC directly in a browser supporting immediate playback of any video/audio codec/container supplied in the WASM payload. The major advantage of a browser based application versus a desktop app is immediate playback of media via hyperlink without the soul crushing limitations Apple imposes on iOS.
A media library written for WASM with supplied codecs would not have that limitation, such that you could have an equivalent to VLC directly in a browser supporting immediate playback of any video/audio codec/container supplied in the WASM payload. The major advantage of a browser based application versus a desktop app is immediate playback of media via hyperlink without the soul crushing limitations Apple imposes on iOS.