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HTML/CSSIntermediate#css#pseudo-elements
HTML/CSSIntermediate#css#cascade#specificity
What does !important do and why should it generally be avoided?
!important overrides normal specificity rules, forcing a declaration to win regardless of selector weight. It should be avoided because it breaks the natural cascade, makes debugging harder, and forces overuse elsewhere to override it, creating a specificity arms race.
Example
.text { color: black !important; }
/* Even a more specific selector below cannot override this without its own !important */
#header .text { color: red; } /* loses */