He was likely using a polyfill. It’s definitely not in the spec and there’s an open discussion about trying to get it added: https://github.com/whatwg/html/issu

He was likely using a polyfill. It’s definitely not in the spec and there’s an open discussion about trying to get it added: https://github.com/whatwg/html/issues/2177 https://github.com/whatwg/html/issues/2177