<div></div>
<p>万维网联盟（W3C） 使用了一套特别的语法来定义 CSS 的属性值，能让所有的 CSS 属性都用。如果你曾看过 CSS 规范，你可能已经见过这套语法了。就像 </p><div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:css;toolbar:false;">border-image-slice</pre><div class="contentsignin">登录后复制</div></div><p> 的语法 ，让我们看看：<br/></p><div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:css;toolbar:false"><'border-image-slice'> = [<number> | <percentage>]{1,4} && fill?</pre><div class="contentsignin">登录后复制</div></div><p>如果你不知道这些符号以及它们如何工作的话，这套语法可能非常难理解。然而，这值得花时间来学。如果你理解 W3C 是如何定义这些属性值的，你就可以理解 W3C CSS 规范 中任意一个了。<br/><br/><img src="https://img.php.cn/upload/article/000/000/194/4b2e0cb3ffe4e9d2c7616cbfbd8d0a35-0.jpg"/ alt="css属性值语法的详细介绍" ><br/><br/></p><h2>巴科斯范式</h2><p>首先，我们看看巴科斯范式（Backus-Naur Form），因为这能帮我们理解 W3C 的属性值语法。<br/><br/>Backus–Naur Form （BNF） 是用来描述计算机语言语法的正式符号集。它被设计得很清晰，所以在语言如何表达方面不会造成二义或者模糊。<br/><br/>最初 Backus-Naur 符号集有很多的扩展与变种在今天都在使用，包括 扩展巴科斯范式（EBNF）和扩充巴克斯范式（ABNF）.<br/><br/>一个 BNF 规范是按下面的形式编写的一套规则：<br/></p><div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:css;toolbar:false"><symbol> ::= __expression__</pre><div class="contentsignin">登录后复制</div></div><p>式子左边通常是一个非终止符，跟着一个 <br/></p><div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:css;toolbar:false;">::=</pre><div class="contentsignin">登录后复制</div></div><p> 符号，代表着“可被换为”。式子右边 </p><div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:plain;">__expression__</pre><div class="contentsignin">登录后复制</div></div><p>由一或多个符号序列组成，这些符号序列被用来推导左侧符号的意义。<br/><br/>BNF 规范从根本上说，“无论左侧式子是什么，也无论右侧式子是什么，左侧的式子都能被右侧的式子替换”。<br/><br/></p><h2>非终止符与终止符</h2><p>非终止符是指能在之后被替换或被分解的符号。在 BNF 中，非终止符通常都在尖角括号中，</p><div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:plain;"><</pre><div class="contentsignin">登录后复制</div></div><p> 与 </p><div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:plain;">></pre><div class="contentsignin">登录后复制</div></div><p>。在下面的例子中，</p><div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:plain;"><integet></pre><div class="contentsignin">登录后复制</div></div><p> 和 </p><div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:plain;"><digit></pre><div class="contentsignin">登录后复制</div></div><p> 是非终止符。<br/></p><div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:css;toolbar:false"><integer> ::= <digit> | <digit><integer></pre><div class="contentsignin">登录后复制</div></div><p>终止符表明这个值不能被替换或者分解。在下面的例子中，所有的数值都是终止符。<br/></p><div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:css;toolbar:false"><digit> ::= 0 | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9</pre><div class="contentsignin">登录后复制</div></div><h2>CSS 属性值语法</h2><p>尽管 W3C CSS 属性值语法是基于 BNF 的概念，它们也有一些不同。像 BNF 的是，它起始于一个非终止符。不像 BNF 的是，它还描述了用在表达式中作为“成分值（component values）”的符号。<br/><br/>在下面的例子中，</p><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:plain;"><line-width></pre><div class="contentsignin">登录后复制</div></div><div class="contentsignin">登录后复制</div></div><p> 是非终止符，而 </p><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:plain;"><length></pre><div class="contentsignin">登录后复制</div></div><div class="contentsignin">登录后复制</div></div><p>，</p><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:plain;">thin</pre><div class="contentsignin">登录后复制</div></div><div class="contentsignin">登录后复制</div></div><p>，</p><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:plain;">medium</pre><div class="contentsignin">登录后复制</div></div><div class="contentsignin">登录后复制</div></div><p> 和 </p><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:plain;">thick</pre><div class="contentsignin">登录后复制</div></div><div class="contentsignin">登录后复制</div></div><p> 是成分值。<br/></p><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:css;toolbar:false"><line-width> = <length> | thin | medium | thick</pre><div class="contentsignin">登录后复制</div></div><div class="contentsignin">登录后复制</div></div><h2>成分值</h2><p>有四种成分值：关键字、基本数据类型、属性数据类型与非数据数据类型。<br/></p><h3>1. 关键字值</h3><p>关键字值不被引号或尖角括号包围。它们可直接作为属性值。因为它们不能再被代替或分解，所以它们是终止符。在下面的例子中，</p><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:plain;">thin</pre><div class="contentsignin">登录后复制</div></div><div class="contentsignin">登录后复制</div></div><p>、</p><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:plain;">medium</pre><div class="contentsignin">登录后复制</div></div><div class="contentsignin">登录后复制</div></div><p> 和 </p><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:plain;">thick</pre><div class="contentsignin">登录后复制</div></div><div class="contentsignin">登录后复制</div></div><p> 都是关键字值。这意味着它们在 CSS 中直接使用。<br/></p><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:css;toolbar:false"><line-width> = <length> | thin | medium | thick</pre><div class="contentsignin">登录后复制</div></div><div class="contentsignin">登录后复制</div></div><h3>2. 基本数据类型</h3><p>基本数据类型定义了一些核心值，如 </p><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:plain;"><length></pre><div class="contentsignin">登录后复制</div></div><div class="contentsignin">登录后复制</div></div><p> 与 </p><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:plain;"><color></pre><div class="contentsignin">登录后复制</div></div><div class="contentsignin">登录后复制</div></div><div class="contentsignin">登录后复制</div></div><div class="contentsignin">登录后复制</div></div><p>。它们是非终止符，因为它们可以被替换成真实的长度或颜色值。在下面的例子中 </p><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:plain;"><color></pre><div class="contentsignin">登录后复制</div></div><div class="contentsignin">登录后复制</div></div><div class="contentsignin">登录后复制</div></div><div class="contentsignin">登录后复制</div></div><p> 是基本数据类型。<br/></p><div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:css;toolbar:false"><'background-color'> = <color></pre><div class="contentsignin">登录后复制</div></div><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:plain;"><color></pre><div class="contentsignin">登录后复制</div></div><div class="contentsignin">登录后复制</div></div><div class="contentsignin">登录后复制</div></div><div class="contentsignin">登录后复制</div></div><p> 可在我们的 CSS 中，通过关键字，扩展关键字，RGB、RGBA、HSL、HSLA，或 </p><div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:plain;">transparent</pre><div class="contentsignin">登录后复制</div></div><p> 关键字，被替换为实际的颜色值。<br/><br/></p><div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:css;toolbar:false">.example { background-color: red; }
.example { background-color: honeydew; }
.example { background-color: rgb(50%,50%,50%); }
.example { background-color: rgba(100%,100%,100%,.5); }
.example { background-color: hsl(280,100%,50%); }
.example { background-color: hsla(280,100%,50%,0.5); }
.example { background-color: transparent; }</pre><div class="contentsignin">登录后复制</div></div><h3>3. 属性数据类型</h3><p>属性数据类型定义了属性实际的名字，是非终止符。它由包含在尖角括号中的属性名（包含引号）定义。在下面的例子中，</p><div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:plain;"><'border-width'></pre><div class="contentsignin">登录后复制</div></div><p> 是属性数据类型。<br/></p><div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:css;toolbar:false"><'border-width'> = <line-width>{1,4}</pre><div class="contentsignin">登录后复制</div></div><p>属性数据类型可作为属性直接出现在我们的 CSS 文件中。在下面的例子中，<br/></p><div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:plain;">border-width</pre><div class="contentsignin">登录后复制</div></div><p> 属性给 </p><div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:plain;">.exmplate</pre><div class="contentsignin">登录后复制</div></div><p> 类定义了 2px 的边框。<br/></p><div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:css;toolbar:false">.example { border-width: 2px; }</pre><div class="contentsignin">登录后复制</div></div><h3>4. 非属性数据类型</h3><p>非属性数据类型并不与属性分享同一个名字，是非终止符。然而，它定义了某个（些）属性的一些层面。例如，</p><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:plain;"><line-width></pre><div class="contentsignin">登录后复制</div></div><div class="contentsignin">登录后复制</div></div><p> 不是个属性，但它是一个定义了各种 </p><div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:plain;"><border></pre><div class="contentsignin">登录后复制</div></div><p> 的数据类型。<br/></p><div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:css;toolbar:false"><line-width> = <length> | thin | medium | thick
<'border-width'> = <line-width>{1,4}</pre><div class="contentsignin">登录后复制</div></div><h2>成分值组合器（Combinator）</h2><p>使用下面的五个方法，成分值能被分配至属性值组合器：<br/></p><h3>1. 相邻值</h3><p>成分值接连而写意味着所有这些值都必须按给定的顺序出现。在下面的例子中，语法列出了三个不同的值：</p><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:plain;">value1</pre><div class="contentsignin">登录后复制</div></div><div class="contentsignin">登录后复制</div></div><div class="contentsignin">登录后复制</div></div><div class="contentsignin">登录后复制</div></div><div class="contentsignin">登录后复制</div></div><div class="contentsignin">登录后复制</div></div><div class="contentsignin">登录后复制</div></div><div class="contentsignin">登录后复制</div></div><p>, </p><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:plain;">value2</pre><div class="contentsignin">登录后复制</div></div><div class="contentsignin">登录后复制</div></div><div class="contentsignin">登录后复制</div></div><div class="contentsignin">登录后复制</div></div><div class="contentsignin">登录后复制</div></div><div class="contentsignin">登录后复制</div></div><p> 与 </p><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:plain;">value3</pre><div class="contentsignin">登录后复制</div></div><div class="contentsignin">登录后复制</div></div><div class="contentsignin">登录后复制</div></div><div class="contentsignin">登录后复制</div></div><div class="contentsignin">登录后复制</div></div><p>。在 CSS 规则中，这三个值必须按照正确的顺序出现才算合法。<br/></p><div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:css;toolbar:false">/* Component arrangement: all in given order */
<'property'> = value1 value2 value3
/* Example */
.example { property: value1 value2 value3; }</pre><div class="contentsignin">登录后复制</div></div><h3>2. 双与符号（&）</h3><p>分开两个或更多成分值的双与符号（</p><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:plain;">&&</pre><div class="contentsignin">登录后复制</div></div><div class="contentsignin">登录后复制</div></div><p>）意味着，这些值必须出现，顺序任意。在下面的例子中，语法列出了两个值，由双与符号分开。下面的 CSS 规则说明了这两个值都得出现但可能是不同的顺序。<br/><br/></p><div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:css;toolbar:false">/* Component arrangement: all, in any order */
<'property'> = value1 && value2
/* Examples */
.example { property: value1 value2; }
.example { property: value2 value1; }</pre><div class="contentsignin">登录后复制</div></div><h3>3. 单管道符号</h3><p>分开两个或更多成分值的单管道符号（</p><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:plain;">|</pre><div class="contentsignin">登录后复制</div></div><div class="contentsignin">登录后复制</div></div><p>）意味着，这些值中只需一个值出现。在下面的例子中，语法列出了三个值，由单管道符号分开。在下面的 CSS 规则中展示了三个可能选项：<br/><br/></p><div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:css;toolbar:false">/* Component arrangement: one of them must occur */
<'property'> = value1 | value2 | value3
/* Examples */
.example { property: value1; }
.example { property: value2; }
.example { property: value3; }</pre><div class="contentsignin">登录后复制</div></div><h3>4. 双管道符号</h3><p>分开两个或更多选择的双管道符号（</p><div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:plain;">||</pre><div class="contentsignin">登录后复制</div></div><p>）意味着，这些值中一个或多个值必须出现，顺序任意。在下面的例子中，语法列出了三个值，由双管道符号分开。在你写 CSS 规则来匹配这个语法时，有大量可选的选择 —— 你可以使用一个，两个或三个值，以任意顺序。<br/><br/></p><div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:css;toolbar:false">/* Component arrangement: one or more in any order */
<'property'> = value1 || value2 || value3
/* Examples */
.example { property: value1; }
.example { property: value2; }
.example { property: value3; }
.example { property: value1 value2; }
.example { property: value1 value2 value3; }
...etc</pre><div class="contentsignin">登录后复制</div></div><h3>5. 中括号</h3><p>包住了两个或更多选择的中括号（</p><div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:plain;">[ ]</pre><div class="contentsignin">登录后复制</div></div><p>）意味着其中的成分值属于一个单独的组。在下面的例子中，语法列出了三个值，但其中两个在中括号中，所以它们属于一个组。所以在 CSS 规则中有两种选择：</p><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:plain;">value1</pre><div class="contentsignin">登录后复制</div></div><div class="contentsignin">登录后复制</div></div><div class="contentsignin">登录后复制</div></div><div class="contentsignin">登录后复制</div></div><div class="contentsignin">登录后复制</div></div><div class="contentsignin">登录后复制</div></div><div class="contentsignin">登录后复制</div></div><div class="contentsignin">登录后复制</div></div><p> 与 </p><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:plain;">value3</pre><div class="contentsignin">登录后复制</div></div><div class="contentsignin">登录后复制</div></div><div class="contentsignin">登录后复制</div></div><div class="contentsignin">登录后复制</div></div><div class="contentsignin">登录后复制</div></div><p> 或 </p><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:plain;">value2</pre><div class="contentsignin">登录后复制</div></div><div class="contentsignin">登录后复制</div></div><div class="contentsignin">登录后复制</div></div><div class="contentsignin">登录后复制</div></div><div class="contentsignin">登录后复制</div></div><div class="contentsignin">登录后复制</div></div><p> 与 </p><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:plain;">value3</pre><div class="contentsignin">登录后复制</div></div><div class="contentsignin">登录后复制</div></div><div class="contentsignin">登录后复制</div></div><div class="contentsignin">登录后复制</div></div><div class="contentsignin">登录后复制</div></div><div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:css;toolbar:false">/* Component arrangement: a single grouping */
<'property'> = [ value1 | value2 ] value3
/* Examples */
.example { property: value1 value3; }
.example { property: value2 value3; }</pre><div class="contentsignin">登录后复制</div></div><h2>成分值累乘器（Multipliers）</h2><p>使用下列 8 个方法之一，成分值也可被重用：<br/></p><h3>1. <div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:plain;">?</pre><div class="contentsignin">登录后复制</div></div><div class="contentsignin">登录后复制</div></div><div class="contentsignin">登录后复制</div></div></h3><p>问号（</p><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:plain;">?</pre><div class="contentsignin">登录后复制</div></div><div class="contentsignin">登录后复制</div></div><div class="contentsignin">登录后复制</div></div><p>）表明其之前的类型，关键字或者组，是可选的且出现零次或一次。在下面的例子中，第二个成分值与一个逗号一起放在了中括号里。放置其后的问号意味着，</p><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:plain;">value1</pre><div class="contentsignin">登录后复制</div></div><div class="contentsignin">登录后复制</div></div><div class="contentsignin">登录后复制</div></div><div class="contentsignin">登录后复制</div></div><div class="contentsignin">登录后复制</div></div><div class="contentsignin">登录后复制</div></div><div class="contentsignin">登录后复制</div></div><div class="contentsignin">登录后复制</div></div><p> 必须出现，但我们也可使用 </p><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:plain;">value1</pre><div class="contentsignin">登录后复制</div></div><div class="contentsignin">登录后复制</div></div><div class="contentsignin">登录后复制</div></div><div class="contentsignin">登录后复制</div></div><div class="contentsignin">登录后复制</div></div><div class="contentsignin">登录后复制</div></div><div class="contentsignin">登录后复制</div></div><div class="contentsignin">登录后复制</div></div><p> 和 </p><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:plain;">value2</pre><div class="contentsignin">登录后复制</div></div><div class="contentsignin">登录后复制</div></div><div class="contentsignin">登录后复制</div></div><div class="contentsignin">登录后复制</div></div><div class="contentsignin">登录后复制</div></div><div class="contentsignin">登录后复制</div></div><p>，以逗号分隔。<br/><br/></p><div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:css;toolbar:false">/* Component multiplier: zero or one time */
<'property'> = value1 [, value2 ]?
/* Examples */
.example { property: value1; }
.example { property: value1, value2; }</pre><div class="contentsignin">登录后复制</div></div><h3>2. <div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:plain;">*</pre><div class="contentsignin">登录后复制</div></div><div class="contentsignin">登录后复制</div></div></h3><p>星号（</p><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:plain;">*</pre><div class="contentsignin">登录后复制</div></div><div class="contentsignin">登录后复制</div></div><p>）表明其之前的类型，关键字或者组出现零次或更多次。在下面的例子中，第二个成分值与一个逗号一起放在了中括号里。放置其后的星号意味着，</p><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:plain;">value1</pre><div class="contentsignin">登录后复制</div></div><div class="contentsignin">登录后复制</div></div><div class="contentsignin">登录后复制</div></div><div class="contentsignin">登录后复制</div></div><div class="contentsignin">登录后复制</div></div><div class="contentsignin">登录后复制</div></div><div class="contentsignin">登录后复制</div></div><div class="contentsignin">登录后复制</div></div><p> 必须出现，但我们也能随我们想地使用 </p><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:plain;">value2</pre><div class="contentsignin">登录后复制</div></div><div class="contentsignin">登录后复制</div></div><div class="contentsignin">登录后复制</div></div><div class="contentsignin">登录后复制</div></div><div class="contentsignin">登录后复制</div></div><div class="contentsignin">登录后复制</div></div><p> 任意次，每个成分值以逗号分隔。<br/><br/></p><div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:css;toolbar:false">/* Component multiplier: zero or more times */
<'property'> = value1 [, <value2> ]*
/* Examples */
.example { property: value1; }
.example { property: value1, <value2>; }
.example { property: value1, <value2>, <value2>; }
.example { property: value1, <value2>, <value2>, <value2>; }
...etc</pre><div class="contentsignin">登录后复制</div></div><h3>3. <div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:plain;">+</pre><div class="contentsignin">登录后复制</div></div><div class="contentsignin">登录后复制</div></div></h3><p>加号（</p><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:plain;">+</pre><div class="contentsignin">登录后复制</div></div><div class="contentsignin">登录后复制</div></div><p>）表明其之前的类型，关键字或者组出现一次或更多次。在下面的例子中，放置于成分值之后的加号意味着该值必须被使用超过一次 —— 无需逗号。<br/><br/></p><div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:css;toolbar:false">/* Component multiplier: one or more times */
<'property'> = <value>+
/* Examples */
.example { property: <value>; }
.example { property: <value> <value>; }
.example { property: <value> <value> <value>; }
...etc</pre><div class="contentsignin">登录后复制</div></div><h3>4. <div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:plain;">{A}</pre><div class="contentsignin">登录后复制</div></div><div class="contentsignin">登录后复制</div></div></h3><p>大括号（</p><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:plain;">{A}</pre><div class="contentsignin">登录后复制</div></div><div class="contentsignin">登录后复制</div></div><p>）中包含一个数字表明其之前的类型，关键字或者组出现 </p><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:plain;">A</pre><div class="contentsignin">登录后复制</div></div><div class="contentsignin">登录后复制</div></div><div class="contentsignin">登录后复制</div></div><p> 次。在下面的例子中，value 的两个实例都必须根据出现才合法。<br/><br/></p><div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:css;toolbar:false">/* Component multiplier: occurs A times */
<'property'> = <value>{2}
/* Examples */
.example { property: <value> <value> ; }</pre><div class="contentsignin">登录后复制</div></div><h3>5. <div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:plain;">{A,B}</pre><div class="contentsignin">登录后复制</div></div><div class="contentsignin">登录后复制</div></div></h3><p>大括号（</p><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:plain;">{A,B}</pre><div class="contentsignin">登录后复制</div></div><div class="contentsignin">登录后复制</div></div><p>）中包含由逗号分开的两个数字表明其之前的类型，关键字或者组出现至少 </p><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:plain;">A</pre><div class="contentsignin">登录后复制</div></div><div class="contentsignin">登录后复制</div></div><div class="contentsignin">登录后复制</div></div><p> 次，至少 </p><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:plain;">B</pre><div class="contentsignin">登录后复制</div></div><div class="contentsignin">登录后复制</div></div><p> 次。在下面的例子中，最少一个、最多三个值肯能被用来定义该属性。这些成分值不以逗号分离。<br/><br/></p><div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:css;toolbar:false">/* Component multiplier: at least A and at most B */
<'property'> = <value>{1,3}
/* Examples */
.example { property: <value>; }
.example { property: <value> <value>; }
.example { property: <value> <value> <value>; }</pre><div class="contentsignin">登录后复制</div></div><h3>6. <div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:plain;">{A,}</pre><div class="contentsignin">登录后复制</div></div><div class="contentsignin">登录后复制</div></div></h3><p>在 </p><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:plain;">{A,}</pre><div class="contentsignin">登录后复制</div></div><div class="contentsignin">登录后复制</div></div><p> 中 </p><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:plain;">B</pre><div class="contentsignin">登录后复制</div></div><div class="contentsignin">登录后复制</div></div><p> 被省去了，这意味着至少有 </p><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:plain;">A</pre><div class="contentsignin">登录后复制</div></div><div class="contentsignin">登录后复制</div></div><div class="contentsignin">登录后复制</div></div><p> 次重复，而没有上限。在下面的例子中，至少需要使用一个成分值，但也可以额外使用任意数量的成分值值。这些成分值不以逗号分离。<br/><br/></p><div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:css;toolbar:false">/* Component multiplier: at least A, with no upper limit */
<'property'> = <value>{1,}
/* Examples */
.example { property: <value>; }
.example { property: <value> <value>; }
.example { property: <value> <value> <value> ; }
...etc</pre><div class="contentsignin">登录后复制</div></div><h3>7. <div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:plain;">#</pre><div class="contentsignin">登录后复制</div></div><div class="contentsignin">登录后复制</div></div><div class="contentsignin">登录后复制</div></div></h3><p>井号（</p><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:plain;">#</pre><div class="contentsignin">登录后复制</div></div><div class="contentsignin">登录后复制</div></div><div class="contentsignin">登录后复制</div></div><p>）表明其之前的类型，关键字或者组出现一次或多次。在下面的例子中，一个或多个成分值可能被使用，这些成分值以逗号分离。<br/><br/></p><div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:css;toolbar:false">/* Component multiplier: one or more, separated by commas */
<'property'> = <value>#
/* Examples */
.example { property: <value>; }
.example { property: <value>, <value>; }
.example { property: <value>, <value>, <value>; }
...etc</pre><div class="contentsignin">登录后复制</div></div><h3>8. <div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:plain;">!</pre><div class="contentsignin">登录后复制</div></div><div class="contentsignin">登录后复制</div></div></h3><p>一个组后的感叹号（</p><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:plain;">!</pre><div class="contentsignin">登录后复制</div></div><div class="contentsignin">登录后复制</div></div><p>）意味着该组是必须的且产生至少一个值。在下面的例子中，</p><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:plain;">value1</pre><div class="contentsignin">登录后复制</div></div><div class="contentsignin">登录后复制</div></div><div class="contentsignin">登录后复制</div></div><div class="contentsignin">登录后复制</div></div><div class="contentsignin">登录后复制</div></div><div class="contentsignin">登录后复制</div></div><div class="contentsignin">登录后复制</div></div><div class="contentsignin">登录后复制</div></div><p> 是必须的，以及一个来自与由 </p><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:plain;">value2</pre><div class="contentsignin">登录后复制</div></div><div class="contentsignin">登录后复制</div></div><div class="contentsignin">登录后复制</div></div><div class="contentsignin">登录后复制</div></div><div class="contentsignin">登录后复制</div></div><div class="contentsignin">登录后复制</div></div><p> 与 </p><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:plain;">value3</pre><div class="contentsignin">登录后复制</div></div><div class="contentsignin">登录后复制</div></div><div class="contentsignin">登录后复制</div></div><div class="contentsignin">登录后复制</div></div><div class="contentsignin">登录后复制</div></div><p> 组成的组的值。该属性只有两个属性值；它们是，</p><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:plain;">value1</pre><div class="contentsignin">登录后复制</div></div><div class="contentsignin">登录后复制</div></div><div class="contentsignin">登录后复制</div></div><div class="contentsignin">登录后复制</div></div><div class="contentsignin">登录后复制</div></div><div class="contentsignin">登录后复制</div></div><div class="contentsignin">登录后复制</div></div><div class="contentsignin">登录后复制</div></div><p>与 </p><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:plain;">value2</pre><div class="contentsignin">登录后复制</div></div><div class="contentsignin">登录后复制</div></div><div class="contentsignin">登录后复制</div></div><div class="contentsignin">登录后复制</div></div><div class="contentsignin">登录后复制</div></div><div class="contentsignin">登录后复制</div></div><p> 或 </p><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:plain;">value1</pre><div class="contentsignin">登录后复制</div></div><div class="contentsignin">登录后复制</div></div><div class="contentsignin">登录后复制</div></div><div class="contentsignin">登录后复制</div></div><div class="contentsignin">登录后复制</div></div><div class="contentsignin">登录后复制</div></div><div class="contentsignin">登录后复制</div></div><div class="contentsignin">登录后复制</div></div><p> 与 </p><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:plain;">value3</pre><div class="contentsignin">登录后复制</div></div><div class="contentsignin">登录后复制</div></div><div class="contentsignin">登录后复制</div></div><div class="contentsignin">登录后复制</div></div><div class="contentsignin">登录后复制</div></div><p>。<br/><br/></p><div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:css;toolbar:false">/* Component multiplier: required group, at least one value */
<'property'> = value1 [ value2 | value3 ]!
/* Examples */
.example { property: value1 value2; }
.example { property: value1 value3; }</pre><div class="contentsignin">登录后复制</div></div><h2>一个例子：<div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:plain;"><'text-shadow'></pre><div class="contentsignin">登录后复制</div></div><div class="contentsignin">登录后复制</div></div> 语法</h2><p>让我们把 </p><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:plain;"><'text-shadow'></pre><div class="contentsignin">登录后复制</div></div><div class="contentsignin">登录后复制</div></div><p> 当作例子观察一番。这是它在规范里的定义：<br/></p><div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:css;toolbar:false"><'text-shadow'> = none | [ <length>{2,3} && <color>? ]#</pre><div class="contentsignin">登录后复制</div></div><p>我们可以拆分这些符号：<br/></p><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:plain;">|</pre><div class="contentsignin">登录后复制</div></div><div class="contentsignin">登录后复制</div></div><p> 表明我们可以使用关键字 </p><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:plain;">none</pre><div class="contentsignin">登录后复制</div></div><div class="contentsignin">登录后复制</div></div><p> 或者一个组<br/><br/></p><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:plain;">#</pre><div class="contentsignin">登录后复制</div></div><div class="contentsignin">登录后复制</div></div><div class="contentsignin">登录后复制</div></div><p> 表明我们可以使用这个组一次或多次，以逗号分割<br/><br/>在组中，</p><div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:plain;">{2,3}</pre><div class="contentsignin">登录后复制</div></div><p> 表明我们可以使用 2 或 3 个长度值<br/><br/></p><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:plain;">&&</pre><div class="contentsignin">登录后复制</div></div><div class="contentsignin">登录后复制</div></div><p> 意味着我们必须包括所有值，但顺序可以任意<br/><br/>有点棘手的是，</p><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:plain;"><color></pre><div class="contentsignin">登录后复制</div></div><div class="contentsignin">登录后复制</div></div><div class="contentsignin">登录后复制</div></div><div class="contentsignin">登录后复制</div></div><p> 后有一个 </p><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:plain;">?</pre><div class="contentsignin">登录后复制</div></div><div class="contentsignin">登录后复制</div></div><div class="contentsignin">登录后复制</div></div><p> ，这意味着其可能出现零次或一次。<br/><br/>用简单的话讲，这也可以被写成：<br/><br/></p><div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:php;toolbar:false">指明了 none 或 一个或多个由逗号分离的组，其中包含了二到三个长度值与一个可选的颜色值。长度值与可选的颜色值可以以任意顺序编写。</pre><div class="contentsignin">登录后复制</div></div><p><br/>这意味着我们能够以很多不同的方式来写 </p><div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:plain;">text-shadow</pre><div class="contentsignin">登录后复制</div></div><p> 属性的值。例如，可以设置其为 </p><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:plain;">none</pre><div class="contentsignin">登录后复制</div></div><div class="contentsignin">登录后复制</div></div><p>:<br/><br/></p><div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:css;toolbar:false">.example { text-shadow: none; }</pre><div class="contentsignin">登录后复制</div></div><p>我们也可以只写两个长度值，这意味着我们将设置阴影水平与竖直方向的便宜，但不会有模糊半径或者颜色值。<br/><br/>因为没有定义模糊半径，将会使用初始值 <br/></p><div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:plain;">0</pre><div class="contentsignin">登录后复制</div></div><p>；所以，该阴影的边缘会很锋利。由于没有定义颜色，所以阴影将使用文本的颜色。<br/><br/></p><div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:css;toolbar:false">.example { text-shadow: 10px 10px; }</pre><div class="contentsignin">登录后复制</div></div><p>如果我们使用了三个长度值，我们将会同时定义阴影的水平与竖直方向的偏移和模糊半径。<br/><br/></p><div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:css;toolbar:false">.example { text-shadow: 10px 10px 10px; }</pre><div class="contentsignin">登录后复制</div></div><p>我们也可以加入颜色，且颜色可以出现在 2 或 3 个长度值的前面或后面。在下面的例子中，red 值可以放在任一长度值的后面。<br/><br/></p><div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:css;toolbar:false">.example { text-shadow: 10px 10px 10px red; }
.example { text-shadow: red 10px 10px 10px; }</pre><div class="contentsignin">登录后复制</div></div><p>最后，我们也能包含多个文本阴影，写作以逗号分隔的组。阴影效果将从前至后分层应用：第一个阴影在最顶层，其它的层在其后。阴影不能覆盖在文本上。在下面的例子中，红色阴影将在绿***阴影的顶上。<br/><br/></p><div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:css;toolbar:false">.example {
text-shadow:
10px 10px red,
-20px -20px 5px lime;
}</pre><div class="contentsignin">登录后复制</div></div><h2>结论</h2>
<p>如果你以写 CSS 为生，了解如何正确地写合法的 CSS 属性值很重要。一旦你了解了不同的值是如何被组合或累乘的，CSS 属性值语法就变得非常容易理解了。然后看 CSS 的规范与写合法的 CSS 都会变得更容易了。<br></p>
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