One upon a time we had Chrome, Edge, Firefox and Safari all running some tests for PRs. It was never robust enough to be truly useful, but we had it until we had to remove Edge and Safari in #9903.

We are now in the process (#13194) of moving Chrome and Firefox to Taskcluster and we'll use the results in #7475.

That still leaves Edge and Safari without any coverage on PRs.

For Edge, there doesn't appear to exist an CI integrated with GitHub where we could run Edge, much less Edge Insider Preview. @thejohnjansen for ideas.

For Safari, it's possible we could use Travis's macOS support, but nobody has tested this. It would also be a fairly different setup from what @jugglinmike has set up. @youennf for ideas.

The value of this would be preventing accidental regressions of tests in Edge and Safari, just like for Chrome and Firefox.