[Sean-Der]

I haven't used WebTransport myself, how much can you control? I wonder how often libwebrtc does stuff that wouldn't be allowed in Javascript. Things like probing and overshooting the window?

Since libwebrtc is all in C++ and 'trusted' it can do stuff like faststart or have error correction/bandwidth estimation that is codec aware.

[kixelated]

There's no probing in any QUIC implementation but it's possible. There's a QUIC extension in the IETF similar to transport-wide-cc but it would still be up to the browser to use it for any upload CC.