Bug 1864818 - Let properties and values ride the trains. r=zrhoffman,firefox-style-system-reviewers

Our implementation is in a reasonably good state. Only test failures
are:

 * Long-standing bug 1533327.
 * A test / spec issue that is covered by D211871 (the animation works
   correctly).
 * TypedOM

So I don't see a reason to keep gating this to nightly-only.

Differential Revision: https://phabricator.services.mozilla.com/D211880

modules/libpref/init/StaticPrefList.yaml

@@ -8897,7 +8897,7 @@
 # Whether Properties and Values is enabled
 - name: layout.css.properties-and-values.enabled
   type: RelaxedAtomicBool
-  value: @IS_NIGHTLY_BUILD@
+  value: true
   mirror: always
   rust: true