Firefox 69: CSS ::cue limitations enforced (#4571)

* Firefox 69: ::cue now limits permitted properties per spec
Previously, `::cue` allowed pretty much anything. Now the allowed
properties are limited per the spec.

Source:
* https://bugzilla.mozilla.org/show_bug.cgi?id=1321488

* Revert "Firefox 69: ::cue now limits permitted properties per spec"

This reverts commit 34712b7. Pushed
to the wrong branch.

* Firefox 69: ::cue now enforces CSS property restrictions
Previously, `::cue` did not enforce the limitations on what CSS
properties you could use in a WebVTT cue. Now those limitations
are properly enforced.

Source:
* https://bugzilla.mozilla.org/show_bug.cgi?id=1321488

css/selectors/cue.json

@@ -17,10 +17,14 @@
             },
             "firefox": {
               "version_added": "55",
-              "notes": "Firefox currently does not support a parameter on <code>::cue</code>."
+              "notes": [
+                "Firefox currently does not support a parameter on <code>::cue</code>.",
+                "Correct limitations to the CSS properties permitted within <code>::cue</code> were implemented in Firefox 69. See <a href='https://wiki.developer.mozilla.org/en-US/docs/Web/CSS/::cue#Permitted_properties'>Permitted properties</a> for a list of the allowed properties."
+              ]
             },
             "firefox_android": {
-              "version_added": "55"
+              "version_added": "55",
+              "notes": "Firefox currently does not support a parameter on <code>::cue</code>."
             },
             "ie": {
               "version_added": false