exp/html: implement Noah's Ark clause

exp/html: implement Noah's Ark clause

Implement the (3-per-family) Noah's Ark clause (i.e. don't put
more than three identical elements on the list of active formatting
elements.

Also, when running tests, sort attributes by name before dumping
them.

Pass 4 additional tests with Noah's Ark clause (including one
that needs attributes to be sorted).

Pass 5 additional, unrelated tests because of sorting attributes.

R=nigeltao, rsc
CC=golang-dev
https://golang.org/cl/6247056