This is not ready for full review, I'm uploading it to get some help on adding a new type
There are some clang-format changes that should be fixed by cbrewster's patch

https://treeherder.mozilla.org/#/jobs?repo=try&revision=b7a6cb1515ba0aae7fe2246a26a961977bebf7c3

https://treeherder.mozilla.org/#/jobs?repo=try&revision=c489f51a016e9f1d1a27d2c2647080417e614456

Triggered Lando (for just part 1 on its own, since the part 0 formatting changes presumably weren't necessary & conflicted with some changes from emilio earlier). But it failed, probably because this needs a rebase after some conflicting changes on autoland.

(see not-too-helpful error message at https://lando.services.mozilla.com/D35831/ )

Backed out for failing test_bug1112014.html

Push that started the failures: https://treeherder.mozilla.org/#/jobs?repo=autoland&resultStatus=pending%2Crunning%2Csuccess%2Ctestfailed%2Cbusted%2Cexception&revision=21d365e43fec9db129707c2c4dd8f05d97bd42bc&group_state=expanded&selectedJob=253782224

Failure log: https://treeherder.mozilla.org/logviewer.html#/jobs?job_id=253782224&repo=autoland&lineNumber=5542

Backout: https://hg.mozilla.org/integration/autoland/rev/6d6b2dbb430b14391ca12e24e92cf8293461538c

https://hg.mozilla.org/mozilla-central/rev/05f05e197b30

When I rebased on central yesterday, the line setting the pref in mochitest.ini was removed. I didn't catch it which is why it failed .test_bug1112014.html. Tests passed after adding it back in.

I've added a note for this property at https://developer.mozilla.org/en-US/docs/Mozilla/Firefox/Experimental_features#text-decoration-skip-ink.

Sebastian