Deprecate <applet>

[foolip]

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.

[domenic]

<applet> also does not work in MS Edge, although they have not removed the HTMLAppletElement interface. In https://twitter.com/JustRogDigiTec/status/631973192068808705 @DigiTec expressed interest in removing it entirely.

[DigiTec]

Applet doesn't function because we disallow the creation of the underlying control, but the element and type still exist. We block all but a fixed set of CLSID's. This is no different than it has been in the past. However, we are also willing to remove the element entirely. My main concerns are with parsing semantics. We want to ensure that when we remove the element itself, that the parser still does the correct thing. We had issues with BGSOUND when we removed it that come to mind.

[domenic]

Thanks @DigiTec! It does indeed look like applet is treated somewhat specially in parsing. Let's try to get some clarification on blink-dev... I'll CC you there.

[zcorpan]

I remember we had some issues in Presto back in 2006 or so because applet did not have (enough) special rules in the HTML parser. I think the Web required <p><applet><p> to nest the ps rather than auto-close the applet. Possibly that is not much of an issue anymore, or it becomes moot if applet shows its fallback anyway, but it seems to me it is somewhat risky and I see little benefit in changing the HTML parser.

[foolip]

Overall usage of <applet> is very low, but it should be possible to measure how often the parser bits actually make a difference if one wanted to remove <applet> fully. But leaving the parser as-is also seems fine to me.

[Ms2ger]

Given the existing interop between browsers and other HTML parsing libraries, I'd prefer not changing the parser in general.

[foolip]

OK, let's do it! Not change the parser, that is.

[foolip]

It's clear from blink-dev that the removal will be attempted at Blink, so I'll make the change to deprecate in the spec now. We can figure out the details along the way, and discuss wherever convenient, including this issue even if closed.