We're the only browser that implements it and we don't even follow the standard for it. I've proposed it be removed from the standard: https://github.com/whatwg/html/pull/3027.
Seems not an urgent thing to me, but it helps us clean up several long-standing issues ... feel free to let me know if we should get it prioritized.
It's been removed from HTML now. web-platform-tests: https://github.com/w3c/web-platform-tests/pull/7349