Part of the spec, as sibling says implemented in Safari, was also implemented in a few specific other engine versions for testing then ripped out. Benefits didn

Part of the spec, as sibling says implemented in Safari, was also implemented in a few specific other engine versions for testing then ripped out. Benefits didn't seem to outweigh the extra complexity: iirc the issue was to do with the semantics -- several engines wanted opt-in for proper tail call elimination (so, a syntax addition), rather than opt-out as per the spec (which seemed to involve a lot more work for what I'm guessing they saw as minimal benefit), so [again iirc] just sorta hit a stalemate.