Supporting soft subs is a complex topic though. Three formats, font embedding, positioning and animations. It's a ton of effort, and anything less than "full featureset support" will mean they don't render how you design them in your full-set editor and local media play. And there will be differences and bugs, at least for a while. I suspect font rendering with various fonts in a media render context will have it's own set of issues.

I also think it'd be nice, but I can totally see how it may not make sense technically (complexity with its burdens vs need) or economically.

Browsers are already absurdly complex though so… maybe? :P