Remove <applet> (except for parsing rules) #454
Description
In #75 we deprecated <applet>. We should consider removing it. Blink has done so and Edge has said they are willing to do so. Is that sufficient, or should we wait for more? Are there tracking bugs for removing it from other browsers?
The main removal here is of HTMLAppletElement, but there are also other places it shows up, e.g. in the window named properties, or document.all.