Deprecate <applet> #75
Description
There's an intent to deprecate and remove <applet> in Blink:
https://groups.google.com/a/chromium.org/d/msg/blink-dev/E9LKKtcKbv8/BTB6dSxACgAJ
If this plan moves forward, we should deprecate it in the spec, and also remove it once it's clear whether removing it is web compatible.