contains selector is true when selector contains &.

Replace every & in a selector with parent.

Combine a parent selector with a nested child selector.

keep_readable_branches sel keeps the branches of sel that a reader accepts, and is None when none does. Nesting composes a parent and a child that are each valid into a selector putting a combinator after a pseudo-element, which CSS Selectors 4 sec. 3.6.5 makes invalid and no engine matches.

drop_dead_nested rule drops from rule's body every nested rule whose selector, composed with the parent's, keep_readable_branches rejects, and every branch of one that keeps others. A dropped rule takes its own body with it, and a conditional block is walked under the same parent, so the body keeps exactly what flattening it would keep. Physically unchanged when nothing is dead.

Merge a pure wrapper rule with its sole nested rule when safe. A merge keep_readable_branches rejects outright leaves the wrapper empty, for the empty-rule pass to drop.

hoist_declaration_runs rule moves each declaration written after a nested statement (CSS Nesting 1 sec. 3.4) back into the rule's own run, as far as it commutes with everything nested above it. A declaration whose property a crossed statement also writes stays where the author put it, so the rule computes the same values while the rest rejoins the declaration passes. Physically unchanged when nothing moves.

Synthesize nested rules from adjacent flat selector chains.

Apply rules to each adjacent run of rule statements.