この翻訳は不完全です。英語から この記事を翻訳 してください。
カスケードは、異なるソースから来るプロパティ値を組み合わせる方法を定義するアルゴリズムです。これはカスケーディングスタイルシートという名前で強調されているように、CSS の中心を占めるものです。この記事では、カスケードとは何か、 CSS 宣言をカスケードする順番、そしてウェブ開発者にどのように影響するかを説明します。
どの CSS エンティティがカスケードに関係するか
Only CSS declarations, that is property/value pairs, participate in the cascade. This means that at-rules containing entities other than declarations, such as a @font-face rule containing descriptors, don't participate in the cascade. In these cases, only the at-rule as a whole participates in the cascade: here, the @font-face identified by its font-family descriptor. If several @font-face rules with the same descriptor are defined, only the most appropriate @font-face, as a whole, is considered.
While the declarations contained in most at-rules — such as those in @media, @document, or @supports — participate in the cascade, declarations contained in @keyframes don't. As with @font-face, only the at-rule as a whole is selected via the cascade algorithm.
Finally, note that @import and @charset obey specific algorithms and aren't affected by the cascade algorithm.
CSS 宣言のオリジン
The CSS cascade algorithm's job is to select CSS declarations in order to determine the correct values for CSS properties. CSS declarations originate from different origins: the user-agent style sheet, the author style sheet, and the user style sheet.
Though style sheets come from these different origins, they overlap in scope; to make this work, the cascade algorithm defines how they interact.
ユーザーエージェントのスタイルシート
The browser has a basic style sheet that gives a default style to any document. These style sheets are named user-agent stylesheets. Some browsers use actual style sheets for this purpose, while others simulate them in code, but the end result is the same.
Some browsers let users modify the user-agent stylesheet. Although some constraints on user-agent stylesheets are set by the HTML specification, browsers still have a lot of latitude: that means that significant differences exist from one browser to another. To simplify the development process, Web developers often use a CSS reset style sheet, forcing common properties values to a known state before beginning to make alterations to suit their specific needs.
作者のスタイルシート
Author stylesheets are the most common type of style sheet. These are style sheets that define styles as part of the design of a given web page or site. The author of the page defines the styles for the document using one or more stylesheets, which define the look and feel of the website — its theme.
ユーザーのスタイルシート
The user (or reader) of the web site can choose to override styles in many browsers using a custom user stylesheet designed to tailor the experience to the user's wishes.
カスケード順
The cascading algorithm determines how to find the value to apply for each property for each document element.
- It first filters all the rules from the different sources to keep only the rules that apply to a given element. That means rules whose selector matches the given element and which are part of an appropriate media at-rule.
- Then it sorts these rules according to their importance, that is, whether or not they are followed by
!important, and by their origin. The cascade is in ascending order, which means that!importantvalues from a user-defined style sheet have precedence over normal values originated from a user-agent style sheet:Origin Importance 1 user agent normal 2 user normal 3 author normal 4 CSS Animations see below 5 author !important6 user !important7 user agent !important - In case of equality, the specificity of a value is considered to choose one or the other.
スタイルのリセット
After your content has finished altering styles, it may find itself in a situation where it needs to restore them to a known state. This may happen in cases of animations, theme changes, and so forth. The CSS property all lets you quickly set (almost) everything in CSS back to a known state.
all lets you opt to immediately restore all properties to any of their initial (default) state, the state inherited from the previous level of the cascade, a specific origin (the user-agent stylesheet, the author stylesheet, or the user stylesheet), or even to clear the values of the properties entirely.
CSS アニメーションとカスケード
CSS animations, using @keyframes at-rules, define animations between states. Keyframes don't cascade, meaning that at any given time CSS takes values from only one single @keyframes, and never mixes multiple ones together.
When several keyframes are appropriate, it chooses the latest defined in the most important document, but never combined all together.
Also note that values within @keyframes at-rules overwrite all normal values but are overwritten by !important values.
例
Let's look at an example involving multiple sources of CSS across the various origins; here we have a user agent style sheet, two author style sheets, a user stylesheet, and inline styles within the HTML:
User-agent CSS:
li { margin-left: 10px }
Author CSS 1:
li { margin-left: 0 } /* This is a reset */
Author CSS 2:
@media screen {
li { margin-left: 3px }
}
@media print {
li { margin-left: 1px }
}
User CSS:
.specific { margin-left: 1em }
HTML:
<ul> <li class="specific">1<sup>st</sup></li> <li>2<sup>nd</sup></li> </ul>
In this case, declarations inside li and .specific rules should apply. No declaration is marked as !important, so the precedence order is author style sheets before user style sheets or user-agent stylesheet.
So three declarations are in competition:
margin-left: 0
margin-left: 3px
margin-left: 1px
The last one is ignored (on a screen), and the first two have the same selector, hence the same specificity. Therefore, it is the last one that is then selected:
margin-left: 3px
Note that the declaration defined in the user CSS, though having a greater specifity, is not chosen as the cascade algorithm is applied before the specifity algorithm.
関連情報
- A very simple introduction to the CSS cascade
- CSS の主要概念: CSS 構文、 詳細度、 継承、 ボックスモデル、 レイアウトモード、 視覚フォーマットモデル、 マージンの相殺、 値 (initial / computed / used / actual)、 値の定義構文、 ショートハンドプロパティ、 置換要素