Nested declarations were getting the location of their parent rule, which would cause issue in the DevTools Inspector (e.g. for InspectorUtils.getRelativeRuleLine). We now track the first declaration of the nested declaration to set it as the location of the rule. This required some changes in upstream cssparser that are vendored in this patch: -5e477ab819-958a3f098aDifferential Revision: https://phabricator.services.mozilla.com/D238911