The Postscript rendering has to be sandboxed, running in a secure, limited context.
At least, that's what NeXT had to do with their Display Postscript. Prior to that it was possible to email someone a postscript file which the WindowServer would try to render for display in the mail window. One such file that went around would, when you clicked on the email, grab all your windows, spin them around the screen, and throw them off.
That's harmless, but Display Postscript included file operations...
Sandboxing would be required.
Delegating an area of display would also need to be part of the security model to support things like third party ads.
Ah, the details…
You'd probably want to write in some other language or tool and use Postscript as an object language, like in the day of printers.
The huge difference is that the evolution of the media would be in the hands of the content creators, not the browser writers/choosers.
I would try this fork of the multiverse.