html: add main and keygen as special element
html: add main and keygen as special element

Also, the isindex element has been remove from specification,
so this commit adds a comment about it.

Change-Id: I79a9b1eb9dae8274e2ca498ab73b2e73521d54e9
Reviewed-on: https://go-review.googlesource.com/64230
Reviewed-by: Tom Bergan <3b77d7da34779f829901a664ac7c87d255b041af@google.com>
Run-TryBot: Tom Bergan <3b77d7da34779f829901a664ac7c87d255b041af@google.com>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>

golang/net