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

Fixes golang/go#25703
Updates golang/go#23071

Change-Id: I09db4c2d07a242cb45c3e37b499c609809dd0b83
Reviewed-on: https://go-review.googlesource.com/120658
Run-TryBot: Kunpei Sakai <namusyaka@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Nigel Tao <nigeltao@golang.org>