CSS basics

Type selectors, e.g. h1 or ::before. Avoid if possible.

# - id selectors ( doka)

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#first {
	color: red;
}

. - class selectors

See also CSS Complex selectors.

Some properties get inherited (mainly text style). I don’t think it’s a good idea to rely on that.

Specificity is a weight that is applied to a given CSS declaration, determined by the number of each selector type in the matching selector.

An algorithms that defines how to combine property values originating from different sources.

Just remember that using !important can be dangerous and that properties declared later (at the bottom) have higher priority.

If elements are in flow, they adhere to rules of inline and block elements. See In Flow and Out of Flow | MDN.