CSS versioning is a mess with countless separate modules, CSS3 literally refers to anything beyond CSS2.1, and there is no such thing as CSS4.
Whether to use progressive enhancement or graceful degradation when providing CSS fallbacks for older browsers.
Error handling in CSS is taken care by CSS parsers where bad or unsupported syntax will be stripped off and recovered gracefully.
Modern CSS has moved away from manual vendor prefixing in favor of automatic prefixing and feature queries.
A CSS behavior when the adjoining margins of two or more elements can combine to form a single margin.
The CSS box model describes the rectangular boxes that are generated for elements in the document tree.
CSS inheritance propagates property values from parent elements to their children, establish the basis for consistent design with little code.
You can achieve a design with different CSS layout systems like Positioned, Flexbox, Grid, or Multi-Column Layout.