Use nesting instead of scoped css - #3939

I checked the desugaring and it's equivalent: inside @scope the implied :scope counts as a pseudo-class (0,1,0), so specificity is unchanged, :scope.sc-wrap → &.sc-wrap is the right translation, and hoisting the .dg-root declarations out of :scope {} is correct. The regex minifier in resources.py is selector-agnostic, so nothing breaks there. No objection to merging.

But: why any 2023-era feature at all? The wrapper does no cascade work here -- the !important declarations do. Written flat, .sc-root .foo { ... } is exactly equivalent and parses in every browser ever shipped. The floors we're choosing between:

Relaxed syntax is required because many nested selectors start with a type (ul.sc-sections, pre.sc-var-data, input.dg-header-in, dl.sc-attrs). In Chrome 112-119 and Safari 16.5-17.1 those rules are dropped while the class-only ones survive -- partially styled, which is the exact broken-layout mode this stylesheet exists to prevent. Firefox ESR 115, still common on institutional Linux, supports neither feature.

So this trades one narrow broken window for another. Flat selectors close the category for good, at the cost of repeating an 8-character prefix in a file we touch twice a year. Since the whole motivation is "someone's browser is too old", I'd take that trade.

If you'd rather keep the nesting, two small asks:

• A comment in both files noting the prefix-by-nesting is deliberate and why @scope is unusable, otherwise this gets modernized back in a year.
• The tested Firefox version in the PR description -- "old and new version of firefox" doesn't pin down what's actually covered.