The Stylo pref "layout.css.servo.enabled" has three states: 1. Stylo support is not built in. (MOZ_STYLO is not defined.) 2. Stylo support is built in but not enabled. (MOZ_STYLO is defined but MOZ_STYLO_ENABLED is not defined.) 3. Stylo support is built in and enabled. (MOZ_STYLO and MOZ_STYLO_ENABLED are both defined.) Defining MOZ_STYLO_ENABLED withing MOZ_STYLO is not supported.

Some tests check the pref: http://searchfox.org/mozilla-central/search?q=css.servo&case=true&path= If it's not defined at all, they may need to be updated to use try blocks or a default value.

Comment on attachment 8880619 [details] Bug 1375699 - Only define the Stylo pref if Stylo support is actually built in. https://reviewboard.mozilla.org/r/151960/#review156964 Looks good, but see jryan's comment.

Thanks. Only one test file needs to be fixed: layout/style/test/animation_utils.js. Waiting for the tests to finish now.

(In reply to Chris Peterson [:cpeterson] from comment #4) > Thanks. Only one test file needs to be fixed: > layout/style/test/animation_utils.js. Waiting for the tests to finish now. I happened to add a few earlier today (before I knew we were planning to not define the pref), so they may not have reached m-c yet: https://hg.mozilla.org/integration/autoland/rev/a7a5b2fc4f0a

Pushed by cpeterson@mozilla.com: https://hg.mozilla.org/integration/mozilla-inbound/rev/3fc6cdc7d80f Only define the Stylo pref if Stylo support is actually built in. r=bholley