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Das Ziel von Cascading Stylesheet (CSS) ist, dem Browser zu ermöglichen, Bestandteile einer Seite mit bestimmten Features, wie Farben, Anordnen, oder Dekorationen, visuell auszuschmücken. Das CSS Syntax spiegelt dieses Ziel wieder und die Grundbausteine sind:

  • Die Eigenschaft ist ein Kennzeichen, that is a human-readable name, das bestimmt, welches Feature gemeint ist.
  • Der Wert beschreibt, wie das Feature vom Browser verarbeitet werden soll. Jede Eigenschaft hat bestimmte Werte die gültig sind, bestimmt durch eine festgelegte Rechtschreibung, sowie eine semantische Bedeutung, die der Browser herbeiführt.

CSS declarations

CSS-Eigenschaften bestimmten Werten zuzuweisen ist die Hauptfunktion von CSS . A property and value pair is called a declaration, and any CSS engine calculates which declarations apply to every single element of a page in order to appropriately lay it out, and to style it.

Both properties and values are case-insensitive by default in CSS. The pair is separated by a colon, ':' (U+003A COLON), and white spaces before, between, and after properties and values, but not necessarily inside, are ignored.

css syntax - declaration.png

Es gibt mehr als 100 verschiedene Eigenschaften in CSS und eine schier unendliche Anzahl von verschiedenen Werten. Aber nicht alle Parungen von Eigenschaften und Werten sind erlaubt and jede Eigenschaft legt fest, was gültige Werte sind. Wenn ein Wert nicht gültig für eine Eigenschaft ist, the declaration is deemed invalid and is wholly ignored by the CSS engine.

CSS declarations blocks

Declarations are grouped in blocks, that is in a structure delimited by an opening brace, '{' (U+007B LEFT CURLY BRACKET), and a closing one, '}' (U+007D RIGHT CURLY BRACKET). Blocks sometimes can be nested, so opening and closing braces must be matched.

css syntax - block.png

Such blocks are naturally called declaration blocks and declarations inside them are separated by a semi-colon, ';' (U+003B SEMICOLON). A declaration block may be empty, that is containing null declaration. White spaces around declarations are ignored. The last declaration of a block doesn't need to be terminated by a semi-colon, though it is often considered good style to do it as it prevents forgetting to add it when extending the block with another declaration.

css syntax - declarations block.png

The content of a CSS declaration block, that is a list of semi-colon-separated declarations, without the initial and closing braces, can be put inside an HTML style attribute.

CSS rulesets

If style sheets could only apply a declaration to each element of a Web page, they would be pretty useless. The real goal is to apply different declarations to different parts of the document.

CSS allows this by associating conditions with declarations blocks. Each (valid) declaration block is preceded by one or more selectors which are conditions selecting some elements of the page. The pair selector group-declarations block is called a ruleset, or often simply a rule.

css syntax - ruleset.png

As an element of the page may be matched by several selectors, and therefore by several rules potentially containing a given property several times, with different values, the CSS standard defines which one has precedence over the other and must be applied: this is called the cascade algorithm.

It is important to note that even if a ruleset characterized by a group of selectors is a kind of shorthand replacing rulesets with a single selector each, this doesn't apply to the validity of the ruleset itself.

This leads to an important consequence: if one single basic selector is invalid, like when using an unknown pseudo-element or pseudo-class, the whole selector is invalid and therefor the entire rule is ignored (as invalid too).

CSS statements

Rulesets are the main building blocks of a style sheet, which often consists of only a big list of them. But there is other information that a Web author wants to convey in the style sheet, like the character set, other external style sheets to import, font face or list counter descriptions and many more. It will use other and specific kinds of statements to do that.

A statement is a building block that begins with any non-space characters and ends at the first closing brace or semi-colon (outside a string, non-escaped and not included into another {}, () or [] pair).

css syntax - statements Venn diag.png

There are two kinds of statements:

  • Rulesets (or rules) that, as seen, associate a collection of CSS declarations to a condition described by a selector.
  • At-rules that start with an at sign, '@' (U+0040 COMMERCIAL AT), followed by an identifier and then continuing up the end of the statement, that is up to the next semi-colon (;) outside of a block, or the end of the next block. Each type of at-rules, defined by the identifier, may have its own internal syntax, and semantics of course. They are used to convey meta-data information (like @charset or @import), conditional information (like @media or @document), or descriptive information (like @font-face).

Any statement which isn't a ruleset or an at-rule is invalid and ignored.

There is another group of statements – the nested statements. These are statements that can be used in a specific subset of at-rules – the conditional group rules. These statements only apply if a specific condition is matched: the @media at-rule content is applied only if the device on which the browser runs matches the expressed condition; the @document at-rule content is applied only if the current page matches some conditions, and so on. In CSS1 and CSS2.1, only rulesets could be used inside conditional group rules. That was very restrictive and this restriction was lifted in CSS Conditionals Level 3. Now, though still experimental and not supported by every browser, conditional group rules can contain a wider range of content: rulesets but also some, but not all, at-rules.

See also

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