[kilian]

This feels like a lay-up, but here you go :) https://polypane.app/docs/emulation/#emulation-or-simulation

Polypane doesn't simulate the specific rendering engine of other browsers, it just pretends to be another browser (which is what emulation is) so you can test that the code you wrote for those browsers (for example, a polyfill) responds well. You'll still need to test in those real browsers to check against their rendering bugs or support gaps. (but something like Polypane portal[1] can make that step much easier)

[1] https://polypane.app/docs/portal/

[jbreckmckye]

Perhaps there's a way you could remotely render a page in Firefox, Safari, etc? And then stream into the viewport.