exp/html: remove "INCOMPLETE" comment

I think that the parser is complete enough to take that warning out.
It passes the test suite.
There may be incompatible API changes, but being in the exp directory
is warning enough for that.

R=nigeltao
CC=golang-dev
https://codereview.appspot.com/7131050

Committer: Nigel Tao <nigeltao@golang.org>