it adds a huge amount of unnecessary complexity for such a simple feature
There is a reason nobody implemented the W3C proposal, If I have the choice between a simple working solution that anyone can describe in a single sentence specification and a hundred page specification with tons of unneeded features that nobody is going to implement ever, I choose the former. I wonder how much hate Mozilla would've gotten for the same feature, probably none.
No, Mozilla would've gotten plenty of hate for potentially breaking sites, and adding features without discussing things with other vendors and Internet bodies. The thing is, we all know that Mozilla can't get away with that, but Google can, given Blink's market dominance.
But various tools have implemented the W3C proposal; just not browsers yet, probably mostly because getting the UI right is tricky, and the risk of breaking things higher. The problem is not in the particular syntax, but the feature as a whole.
There is a reason nobody implemented the W3C proposal, If I have the choice between a simple working solution that anyone can describe in a single sentence specification and a hundred page specification with tons of unneeded features that nobody is going to implement ever, I choose the former. I wonder how much hate Mozilla would've gotten for the same feature, probably none.