By proceeding without distinguishing namespace, inconsistency will
occur.
This commit makes the method distinguish the HTML namespace.

Fixes golang/go#27846

Change-Id: I8269f670240c0fe31162a16fbe1ac23acacec00f
Reviewed-on: https://go-review.googlesource.com/c/159397
Run-TryBot: Kunpei Sakai <namusyaka@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Nigel Tao <nigeltao@golang.org>