Catherine Gluckstein is Head of Product & Strategy at Xbox Cloud Gaming, and yesterday she Tweeted this:
We worked closely with our friends at Valve to support xcloud #XboxGamePass through Microsoft Edge for the Steam Deck.
She attached this picure:
And also included a link to the Microsoft Edge subreddit with instructions on how to set it up:
Those Reddit instructions even include artwork (icon, banner) to make it look nicely integrated.
Pretty cool stuff if you ask me.