Seems that Firefox are planning to support it. Latest discussion on Bugzilla was in May of this year (https://bugzilla.mozilla.org/show_bug.cgi?id=888320 https:

Seems that Firefox are planning to support it. Latest discussion on Bugzilla was in May of this year (https://bugzilla.mozilla.org/show_bug.cgi?id=888320 https://bugzilla.mozilla.org/show_bug.cgi?id=888320) where they were discussing implementation details. No news from Safari.