Parse alternative text for the content property.
This doesn't yet expose it to a11y but that will be done by the a11y
folks, since this blocks some of the a11y interop test-cases.
Modify the tests to not hit the network, and make -moz-alt-content not
exposed to content (we only need it for UA stylesheets).
Differential Revision: https://phabricator.services.mozilla.com/D209690
bugzilla-url: https://bugzilla.mozilla.org/show_bug.cgi?id=1281158
gecko-commit: df46563babe25e025ebca00df6e7e0ffa4f22b9f
gecko-reviewers: dshin