Syntax question in font-face declarations for variable fonts in css2 API

Hi there,

In digging in to the new API (announcement here for those who might not have seen it: https://codepen.io/nlwilliams/full/JjPJewp), I noticed that the `@font-face` syntax is a little out of sync with the spec. Not sure if there's a reason for this, so wanted to pose the question.

When specifying the format, it should be `format('woff2-variations')`, but the CSS being served specifices `format('woff2')`. It would be good to make sure it's following the spec, as others will look to this implementation to learn how to use them.

Example of current syntax:

```
@font-face {
font-family: 'Crimson Pro';
font-style: normal;
font-weight: 200 900;
src: url(https://fonts.gstatic.com/s/crimsonpro/v7/q5uDsoa5M_tv7IihmnkabARVoYF6CsKjnlQ.woff2) format('woff2');
unicode-range: U+0100-024F, U+0259, U+1E00-1EFF, U+2020, U+20A0-20AB, U+20AD-20CF, U+2113, U+2C60-2C7F, U+A720-A7FF;
}
```

should likely be:

```
@font-face {
font-family: 'Crimson Pro';
font-style: normal;
font-weight: 200 900;
src: url(https://fonts.gstatic.com/s/crimsonpro/v7/q5uDsoa5M_tv7IihmnkabARVoYF6CsKjnlQ.woff2) format('woff2-variations');
unicode-range: U+0100-024F, U+0259, U+1E00-1EFF, U+2020, U+20A0-20AB, U+20AD-20CF, U+2113, U+2C60-2C7F, U+A720-A7FF;
}
```