Firefox supports it. You're aware Firefox is OSS right?
That said, at least this css grid garden has a elegant fallback. Most CSS grid sites don't. Or worse, they fall back to 'mobile' sites.
You could modify Firefox source code to let you run whatever add-on you wish. With Firefox whatever changes you want are limited only by your willingness to implement them (either by doing the work yourself or working with others to get those changes made).
You can even distribute the improved Firefox variant browser to others and help the community. Hacker News recently had a story about some hackers doing exactly this with a variant they call Librefox. Other examples of Firefox variants include IceCat and the Tor Browser.