because desktop Steam is technically browser with specific website opened.

The Steam client does use Chromium Embedded Framework for its interface, but there's a lot more to it than just that. For example, Steam Input, Steam Overlay, Steamworks, and Proton (which itself is a collection of of nontrivial components).

If you just want an alternative launcher, there is Lutris, but there is no stand-alone Steam client alternative as far as I know.

It might be interesting to see how much functionality could be replicated in an open-source client. Some components, like DXVK and a web engine, are readily available. Others, like Steamworks, are not. SteamDB shows that it's possible to inspect Steam's game repositories, but actually downloading from them without Steam (or steamcmd) might be challenging. Goldberg Emulator shows that it's possible to fool some games into thinking Steam is running, but that's not enough to run games that include DRM.

Anyone attempting this would have to weigh the time they spend reverse engineering and re-implementing against the fact that Steam can always change its internal services, rendering all that time and effort wasted. And, of course, there would always be a risk that anyone using it (without explicit permission from Valve) might have their account banned.