Support `@starting-style`

[nikeee]

Using the relatively new @starting-style yields a warning:

Docs:
https://developer.mozilla.org/en-US/docs/Web/CSS/@starting-style

Spec:
https://drafts.csswg.org/css-transitions-2/#defining-before-change-style-the-starting-style-rule

CanIUse: https://caniuse.com/mdn-css_at-rules_starting-style (every major browser supports it)