Confirm that the customizer blocks IE<=9, or fix it so that it does.

[cvrebert]

Refs #12773 (comment) & #12617.

[cvrebert]

Confirmed that IE9 is properly blocked.
IE8 however, runs into a fatal JS error.

[cvrebert]

The error seems to be in less.js's initialization.
less.js doesn't support IE8 in the first place:

Less only supports running on modern browsers (recent versions of Chrome, Firefox, Safari and IE).

[XhmikosR]

So basically, there's no IE8 support in customizer.? I remember I had issues when testing stuff, but it was a long time ago.

[cvrebert]

Well, right, as the issue title says, we can't support IE8.
The difference is that with IE8 we crash without a friendly error message; on the other problematic browsers, we show a friendly error message and the JS doesn't outright crash.

[XhmikosR]

Yeah, it's because of less.

So I guess we should explicitly nuke IE 8 too for customizer?

[cvrebert]

Well, ideally yes, but because it goes wrong so early, it's a bit harder than the others.
Maybe use an IE conditional HTML comment?

[XhmikosR]

That should work, yeah.

[cvrebert]

Closing in favor of #13626.