Advice for unicode-range

[svgeesus]

In Opt-In Mechanism a CSS snippet is given to request an IFT font.

There is no mention of unicode-range and I can imagine content authors coming to one of three conclusions here:

1. This is a subset font, so specify the unicode-range or the subset.
2. This is an IFT font, so specify the unicode-range for the fully expanded font
3. This is an IFT font, so could contain anything, so use the initial value, U+0-10FFFF

I think option 1 is clearly wrong, because CSS will never use a font outside the declared unicode-range. So we should explicitly say not to do that.

I think option 2 is correct, but the user may not have convenient access to the fully expanded font to run tooling that deduces the unicode-range.