If it’s washed out, it’s either that there’s a mismatch in pixel format, like u/scutiger was saying or you might be used to oversaturated sRGB, as by default displays are set to stretch the input to their entire range (which if it’s HDR should be close to DCI-P3). sRGB colors shouldn’t look different when displayed in HDR, unless something is misconfigured.

Yea, it works pretty well. I’ve mostly used it for games, but firefox recently added support for HDR video and it seems to work with no issues. mpv I think has had HDR for quite a bit.
I’m not sure what RTX HDR does, as I only used an AMD card with my HDR display, but there are Reshade shaders that should implement something similar and there’s RenoDX (a Reshade addon) which patches games to retrofit real HDR. mpv should have something similar, but I don’t think there’s anything for YouTube.

Edit: mpv’s inverse tonemapping does work for YouTube videos, if you are OK with playing them outside the browser.

Didn’t know that! Last time I tried it had the same issues as normal proton, I think. Now just setting PROTON_ENABLE_WAYLAND works. Thanks.