CSS: make nested rules useful when working in Flat mode

Description

Nested rules can be used in flat translation mode, but the current behavior is not useful.
Make it so that nested rules are exception to the base, leaving the base rule to field for all cases not matched by its nested rules (but without activating full cascading, the nested rules are still executed sequentially as per Flat mode).

Environment

None

Status

Assignee

Andrea Aime

Reporter

Andrea Aime

Triage

None

Components

Fix versions

Priority

Medium
Configure