!important

6.4.2 !important rules

CSS attempts to create a balance of power between author and user style sheets. By default, rules in an author's style sheet override those in a user's style sheet (see cascade rule 3).

cssはauthorとuserでバランスを取ろうとする。 デフォだとauthor > user になる。

However, for balance, an "!important" declaration (the delimiter token "!" and keyword "important" follow the declaration) takes precedence over a normal declaration. Both author and user style sheets may contain "!important" declarations, and user "!important" rules override author "!important" rules. This CSS feature improves accessibility of documents by giving users with special requirements (large fonts, color combinations, etc.) control over presentation.

user, author 両方 !importantだった場合 author < user

Declaring a shorthand property (e.g., 'background') to be "!important" is equivalent to declaring all of its sub-properties to be "!important".

backgroundとかにimportantとか使うとその子の要素にも影響する

The first rule in the user's style sheet in the following example contains an "!important" declaration, which overrides the corresponding declaration in the author's style sheet. The second declaration will also win due to being marked "!important". However, the third rule in the user's style sheet is not "!important" and will therefore lose to the second rule in the author's style sheet (which happens to set style on a shorthand property). Also, the third author rule will lose to the second author rule since the second rule is "!important". This shows that "!important" declarations have a function also within author style sheets.

/* From the user's style sheet */
p { text-indent: 1em ! important }
p { font-style: italic ! important }
p { font-size: 18pt }

/* From the author's style sheet */
p { text-indent: 1.5em !important }
p { font: normal 12pt sans-serif !important }
p { font-size: 24pt }

参照したドキュメントは、CSS 2.1のものだった。 https://www.w3.org/TR/CSS2/cascade.html

こっちはCSS3の最新のドキュメント https://www.w3.org/TR/css3-roadmap/

だから!importantは非推奨