See:
https://html.spec.whatwg.org/multipage/parsing.html#parsing-main-intemplate

Updates golang/go#23071

Change-Id: I36529b7cf5d2adf159ed5c471fba9f67890b7eb9
Reviewed-on: https://go-review.googlesource.com/94838
Run-TryBot: Kunpei Sakai <namusyaka@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Nigel Tao <nigeltao@golang.org>