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<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN"><html lang="en"><HEAD><meta http-equiv="Content-Type" content="text/html; charset=ISO-8859-1"><TITLE>Conformance: requirements and recommendations</TITLE><link rel="stylesheet" href="style/default.css" type="text/css"><link rel="prev" href="intro.html"><link rel="next" href="syndata.html"><link rel="contents" href="cover.html#minitoc"><link rel="CSS-properties" href="propidx.html" title="properties"><link rel="index" href="indexlist.html" title="index"></HEAD><BODY><div class="navbar" align="center"><p><a href="intro.html">previous</a> <a href="syndata.html">next</a> <a href="cover.html#minitoc">contents</a> <a href="propidx.html">properties</a> <a href="indexlist.html">index</a> </div><hr class="navbar"><H1 align="center">3 Conformance: Requirements and Recommendations</H1><div class="subtoc"><p><strong>Contents</strong> <ul class="toc"> <li class="tocline2"><a href="conform.html#q1" class="tocxref">3.1 Definitions</a> <li class="tocline2"><a href="conform.html#conformance" class="tocxref">3.2 Conformance</a> <li class="tocline2"><a href="conform.html#q3" class="tocxref">3.3 Error conditions</a> <li class="tocline2"><a href="conform.html#text-css" class="tocxref">3.4 <span class="index-def" title="text/css">The text/css content type</span></a> </ul></div><H2><a name="q1">3.1 Definitions</a></H2><p> In this section, we begin the formal specification of CSS2,starting with the contract between authors, users, and implementers.<P>The key words <a name="x0"><span class="index-def" title="MUST">"MUST"</span></a>,<a name="x1"><span class="index-def" title="MUST NOT">"MUST NOT"</span></a>, <a name="x2"><spanclass="index-def" title="REQUIRED">"REQUIRED"</span></a>, <a name="x3"><spanclass="index-def" title="SHALL">"SHALL"</span></a>, <a name="x4"><spanclass="index-def" title="SHALL NOT">"SHALL NOT"</span></a>, <a name="x5"><spanclass="index-def" title="SHOULD">"SHOULD"</span></a>, <a name="x6"><spanclass="index-def" title="SHOULD NOT">"SHOULD NOT"</span></a>, <a name="x7"><spanclass="index-def" title="RECOMMENDED">"RECOMMENDED"</span></a>, <a name="x8"><spanclass="index-def" title="MAY">"MAY"</span></a>, and <a name="x9"><spanclass="index-def" title="OPTIONAL">"OPTIONAL"</span></a> in this documentare to be interpreted as described in RFC 2119 (see <a href="refs.html#ref-RFC2119" rel="biblioentry" class="noxref"><span class="normref">[RFC2119]</span></a>).However, for readability, these words do not appear in all uppercaseletters in this specification.<P>At times, this specification recommends good practicefor authors and user agents. These recommendations are not normativeand conformance with this specification does not depend on theirrealization. These recommendations contain the expression "Werecommend ...", "This specification recommends ...", or some similarwording.</P><dl><dt><strong><a name="x10"><span class="index-def" title="style sheet">Stylesheet</span></a></strong><dd>A set of statements that specify presentation of a document.<P>Style sheets may have three different origins: <ahref="#author">author</a>, <a href="#user">user</a>, and <ahref="#user-agent">user agent</a>. The interaction of these sources isdescribed in the section on <a href="cascade.html">cascading andinheritance</a>.<dt><strong><a name="valid-style-sheet">Valid style sheet</a></strong><dd>The <a name="x11"><span class="index-def" title="valid stylesheet">validity</span></a> of a style sheet depends on the level of CSSused for the style sheet. All valid CSS1 style sheets are valid CSS2style sheets. However, some <ahref="changes.html#changes-from-css1">changes from CSS1</a> mean thata few CSS1 style sheets will have slightly different semantics inCSS2.<P>A valid CSS2 style sheet must be written according to the <ahref="grammar.html">grammar of CSS2</a>. Furthermore, it must containonly at-rules, property names, and property values defined in thisspecification. An <a name="x12"><span class="index-def"title="illegal"><strong>illegal</strong></span></a> (invalid) at-rule,property name, or property value is one that is not valid.<dt><strong><a name="x13"><span class="index-def" title="source document">Sourcedocument</span></a></strong><dd>The document to which one or more style sheets refer. This isencoded in some language that represents the document as a tree of <ahref="#element">elements</a>. Each element consists of a name thatidentifies the type of element, optionally a number of <ahref="#attribute">attributes</a>, and a (possibly empty) <ahref="#content">content</a>.</dd><dt><strong><span class="index-def" title="document language"><aname="doclanguage">Document language</a></span></strong><dd>The encoding language of the source document (e.g., HTML or anXML application).<dt><strong><span class="index-def" title="element"><aname="element">Element</a></span></strong><dd>(An SGML term, see <a href="refs.html#ref-ISO8879" rel="biblioentry" class="noxref"><span class="normref">[ISO8879]</span></a>.) The primary syntactic constructsof the document language. Most CSS style sheet rules use the names ofthese elements (such as "P", "TABLE", and "OL" for HTML) to specifyrendering information for them.<dt><strong><span class="index-def" title="replaced element"><a name="replaced-element">Replacedelement</a></span></strong><dd>An element for which the CSS formatter knows only the intrinsicdimensions. In HTML, IMG, INPUT, TEXTAREA, SELECT, and OBJECT elementscan be examples of replaced elements. For example, the content of theIMG element is often replaced by the image that the "src" attributedesignates. CSS does not define how the intrinsic dimensions arefound.<dt><strong><span class="index-def" title="intrinsicdimensions"><a name="intrinsic">Intrinsic dimensions</a></span></strong><dd>The width and height as defined by the element itself, not imposedby the surroundings. In CSS2 it is assumed that all replaced elements-- and only replaced elements -- come with intrinsic dimensions.<dt><strong><span class="index-def" title="attribute"><aname="attribute">Attribute</a></span></strong><dd>A value associated with an element, consisting of a name, and anassociated (textual) value.<dt><strong><span class="index-def" title="content"><aname="content">Content</a></span></strong><dd>The content associated with an element in the source document; notall elements have content in which case they are called <strong><a name="x20"><spanclass="index-def" title="empty">empty</span></a></strong>. The content ofan element may include text, and it may include a number ofsub-elements, in which case the element is called the <strong><a name="x21"><spanclass="index-def" title="parent">parent</span></a></strong> of thosesub-elements.<dt><strong><span class="index-def" title="renderedcontent|content::rendered"><a name="rendered-content">Renderedcontent</a></span></strong><dd>The content of an element after the rendering that applies to itaccording to the relevant style sheets has been applied. The renderedcontent of a <a href="#replaced-element">replaced element</a> comesfrom outside the source document. Rendered content may also bealternate text for an element (e.g., the value of the HTML "alt"attribute), and may include items inserted implicitly or explicitly bythe style sheet, such as bullets, numbering, etc.<dt><strong><span class="index-def" title="document tree"><a name="doctree">Documenttree</a></span></strong><dd>The tree of elements encoded in the source document. Each elementin this tree has exactly one parent, with the exception of the<strong><span class="index-def" title="root"><aname="root">root</a></span></strong> element, which has none.<dt><strong><a name="x26"><span class="index-def"title="child">Child</span></a></strong><dd>An element A is called the child of element B if an only if B isthe parent of A.<dt><strong><a name="x27"><span class="index-def"title="descendant">Descendant</span></a></strong><dd>An element A is called a descendant of an element B, if either (1)A is a child of B, or (2) A is the child of some element C that is adescendant of B.<dt><strong><a name="x28"><span class="index-def"title="ancestor">Ancestor</span></a></strong><dd>An element A is called an ancestor of an element B, if and only ifB is a descendant of A.⌨️ 快捷键说明
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