That's a good question; I'm in the 'external' camp. But other browsers such as Edge and Chrome have built-in support, so there's a percentage of Mozilla managem

That's a good question; I'm in the 'external' camp. But other browsers such as Edge and Chrome have built-in support, so there's a percentage of Mozilla management and user base that would regard a browser as feature incomplete if it didn't have it. At least, once the scaffolding is done, they can hand off development to the Chrome team and thence focus on the core browser issues of implementing HTML 5.x