The built-in Microsoft Edge web browser on the Xbox Series S and Series X consoles can now competently run emulators and ROMs.
According to Modern Vintage Gamer, the updated Edge browser on Xbox Series S/X consoles is now a great way to play retro ROMs and emulators without having download the console's Dev Mode app. This means you can run free javascript-based games natively within the browser without any downloads, including old-school MS-DOS games.
Some emulators like N64 js require you to load the ROMs directly and the Edge browser doesn't allow access to console files, so as MVG notes, you'll have to host the files from another website. Games from the Nintendo 64 and PlayStation 1 era seem to run flawlessly without FPS drops and hitches that can be found in other emulators--it's just seamless play without much setup involved.
The Series X/S' browser support remains one of the biggest advantages the consoles have over Sony's PlayStation 5, which does have a web browser, but it's not directly accessible. The inclusion of Dev Mode is also a huge plus in Microsoft's favor and literally allows gamers to load up specific programs and run them on the system--including emulators.
We managed to get old-school games like Diablo 1 on PS1 running on the Xbox Series X, and were also able to import save files from our PC over to the console as well.