To prevent scrolling artifacts

Closes #3091

Closes #2323

[attr, calc, counter, counters, linear-gradient, matrix, matrix3d,
perspective, radial-gradient, repeating-linear-gradient,
repeating-radial-gradient, rotate, rotate3d, rotateX, rotateY, rotateZ,
scale, scale3d, scaleX, scaleY, scaleZ, skew, skewX, skewY, symbols,
translate, translate3d, translateX, translateY, translateZ, var]

And don't call delete on the state object.

Issue #3084

Add support for @counter-style blocks by making them and @font-face
blocks match a new "restricted_atBlock" state based on "font_face"

See https://bugzilla.mozilla.org/show_bug.cgi?id=1088804

...  for getting information about collapsed spans

Issue #3079

So that the helper mechanism can meaningfully be used to register
asynchronous linters.

Closes #3078

Issue #2064
Issue #3074

Issue #3067

Teach it about @document in a more general way than my first attempt,
and also do @supports the same, by treating both them and @media as
atBlocks with shared word parsing

Issue #3066

Add support for the @supports rule

Issue #3064