This matches what the spec[1] says for font-style: > Animation type: by computed value type; normal animates as oblique 0deg A bunch of WPT tests for font-style animation are landing in https://github.com/web-platform-tests/wpt/pull/37570. Current Gecko passes 66/129 of the testcases there; with this patch applied it passes all the tests. [1] https://drafts.csswg.org/css-fonts-4/#font-style-prop Differential Revision: https://phabricator.services.mozilla.com/D166128
