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CSS优先级规则.html
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CSS优先级规则.html
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class="posts-expand"><article class="post post-type-normal" itemscope itemtype="http://schema.org/Article"><link itemprop="mainEntityOfPage" href="http://mrbird.cc/CSS优先级规则.html"><span hidden itemprop="author" itemscope itemtype="http://schema.org/Person"><meta itemprop="name" content="MrBird"><meta itemprop="description" content=""><meta itemprop="image" content="/images/blogImage.jpg"></span><span hidden itemprop="publisher" itemscope itemtype="http://schema.org/Organization"><meta itemprop="name" content="MrBird"></span><header class="post-header"><h2 class="post-title" itemprop="name headline">CSS优先级规则</h2><div class="post-meta"><span class="post-time"><span class="post-meta-item-icon"><i class="fa fa-calendar-o"></i> </span><span class="post-meta-item-text"></span> <time title="创建于" itemprop="dateCreated datePublished" datetime="2017-04-08T17:30:15+08:00">2017-04-08 </time></span><span></span> <span class="post-meta-divider">|</span> <span class="page-pv"><i class="fa fa-laptop"></i> Visit count <span class="busuanzi-value" id="busuanzi_value_page_pv"></span></span></div></header><div class="post-body" itemprop="articleBody"><h2 id="特殊性"><a href="#特殊性" class="headerlink" title="特殊性"></a>特殊性</h2><p>如果一个元素有两个以上冲突的属性声明，那么特殊性（specificity）高的声明将会胜出。特殊性的值表述为四个部分，如<code>0,0,0,0</code>，具体规则：</p><p>1.选择器中给定的各个ID属性值，加<code>0,1,0,0</code>。</p><p>2.选择器中给定的各个类属型值，属性选择和伪类，加<code>0,0,1,0</code>。</p><p>3.选择器中给定的各个元素和伪元素，加<code>0,0,0,1</code>。</p><p>4.通配符对特殊性没有任何贡献，加<code>0,0,0,0</code>。<a id="more"></a></p><p>比如：</p><figure class="highlight css"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br></pre></td><td class="code"><pre><span class="line"><span class="selector-tag">h1</span> {<span class="attribute">color</span>: red;} <span class="comment">/* specificity = 0,0,0,1 */</span></span><br><span class="line"><span class="selector-tag">p</span> <span class="selector-tag">em</span> {<span class="attribute">color</span>: purple;} <span class="comment">/* specificity = 0,0,0,2 */</span></span><br><span class="line">*<span class="selector-class">.grape</span> {<span class="attribute">color</span>: purple;} <span class="comment">/* specificity = 0,0,1,0 */</span></span><br><span class="line"><span class="selector-class">.bright</span> {<span class="attribute">color</span>: purple;} <span class="comment">/* specificity = 0,0,0,1 */</span></span><br><span class="line"><span class="selector-tag">p</span><span class="selector-class">.bright</span> <span class="selector-tag">em</span><span class="selector-class">.dark</span> {<span class="attribute">color</span>: maroon;} <span class="comment">/* specificity = 0,0,2,2 */</span></span><br><span class="line"><span class="selector-id">#drop-down</span> {<span class="attribute">color</span>: green;} <span class="comment">/* specificity = 0,1,0,0 */</span></span><br><span class="line"><span class="selector-tag">div</span><span class="selector-id">#sidebar</span> *<span class="selector-attr">[href]</span> {<span class="attribute">color</span>: black;} <span class="comment">/* specificity = 0,1,1,1 */</span></span><br></pre></td></tr></table></figure><p></p><div class="note danger"><p>特殊性大小的比较规则为从左到右，数值大的越大。</p></div><p>举个例子，考虑有如下规则的html：</p><figure class="highlight html"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br></pre></td><td class="code"><pre><span class="line"><span class="tag"><<span class="name">style</span>></span><span class="undefined"></span></span><br><span class="line"><span class="css"> <span class="selector-tag">h1</span> + <span class="selector-tag">p</span> {<span class="attribute">color</span>: black; <span class="attribute">font-style</span>: italic} <span class="comment">/* specificity = 0,0,0,2 */</span></span></span><br><span class="line"><span class="css"> <span class="selector-tag">p</span> {<span class="attribute">color</span>: gray; <span class="attribute">background</span>: white; <span class="attribute">font-style</span>: normal} <span class="comment">/* specificity = 0,0,0,1 */</span></span></span><br><span class="line"><span class="css"> *<span class="selector-class">.aside</span> {<span class="attribute">color</span>: black; <span class="attribute">background</span>: silver} <span class="comment">/* specificity = 0,0,1,0 */</span></span></span><br><span class="line"><span class="undefined"></span><span class="tag"></<span class="name">style</span>></span></span><br><span class="line"><span class="tag"><<span class="name">body</span>></span></span><br><span class="line"> <span class="tag"><<span class="name">h1</span>></span>Greeting!<span class="tag"></<span class="name">h1</span>></span></span><br><span class="line"> <span class="tag"><<span class="name">p</span> <span class="attr">class</span>=<span class="string">"aside"</span>></span>It's a fine way to start a day.<span class="tag"></<span class="name">p</span>></span></span><br><span class="line"> <span class="tag"><<span class="name">p</span>></span>There are many ways to greet a man.<span class="tag"></<span class="name">p</span>></span></span><br><span class="line"> <span class="tag"><<span class="name">h1</span>></span>Salutations<span class="tag"></<span class="name">h1</span>></span></span><br><span class="line"> <span class="tag"><<span class="name">p</span>></span>There is nothing finer than a hearty welcome from ...<span class="tag"></<span class="name">p</span>></span></span><br><span class="line"> <span class="tag"><<span class="name">p</span> <span class="attr">class</span>=<span class="string">"aside"</span>></span>Although a thick and juicy hamburger with...<span class="tag"></<span class="name">p</span>></span></span><br><span class="line"><span class="tag"></<span class="name">body</span>></span></span><br></pre></td></tr></table></figure><p></p><p>最终显示如下图所示：</p><p><img src="img/12850410-file_1491624696068_151ff.png" alt="12850410-file_1491624696068_151ff.png"></p><h2 id="内联样式特殊性"><a href="#内联样式特殊性" class="headerlink" title="内联样式特殊性"></a>内联样式特殊性</h2><p>特殊性值的第一位是为内联样式保留的，每个内联样式的特殊性都为<code>1,0,0,0</code>，比如下面的<code>h1</code>元素将显示为绿色：</p><figure class="highlight html"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br></pre></td><td class="code"><pre><span class="line">h1#meadow {color: red}</span><br><span class="line"><span class="tag"><<span class="name">h1</span> <span class="attr">id</span>=<span class="string">"meadow"</span> <span class="attr">style</span>=<span class="string">"color: green;"</span>></span>The title<span class="tag"></<span class="name">h1</span>></span></span><br></pre></td></tr></table></figure><p></p><h2 id="重要性"><a href="#重要性" class="headerlink" title="重要性"></a>重要性</h2><p>对于某个非常重要的声明，可以在这些声明的结束分号前插入<code>!important</code>。如果一个非重要声明和重要声明冲突，胜出的总是重要声明。</p><p>如：</p><figure class="highlight html"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br></pre></td><td class="code"><pre><span class="line"><span class="tag"><<span class="name">style</span>></span><span class="undefined"></span></span><br><span class="line"><span class="css"> <span class="selector-tag">h1</span> {<span class="attribute">color</span>: green <span class="meta">!important</span>;}</span></span><br><span class="line"><span class="css"> <span class="selector-tag">h1</span><span class="selector-class">.title</span> {<span class="attribute">color</span>: red}</span></span><br><span class="line"><span class="undefined"></span><span class="tag"></<span class="name">style</span>></span></span><br><span class="line"><span class="tag"><<span class="name">body</span>></span></span><br><span class="line"> <span class="tag"><<span class="name">h1</span> <span class="attr">class</span>=<span class="string">"title"</span>></span>Greeting!<span class="tag"></<span class="name">h1</span>></span></span><br><span class="line"><span class="tag"></<span class="name">body</span>></span></span><br></pre></td></tr></table></figure><p></p><p>最终<code>h1</code>为绿色。</p><h2 id="继承"><a href="#继承" class="headerlink" title="继承"></a>继承</h2><p>对于继承的值没有特殊性，而非0特殊性。举个例子：</p><figure class="highlight html"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br></pre></td><td class="code"><pre><span class="line"><span class="tag"><<span class="name">style</span>></span><span class="undefined"></span></span><br><span class="line"><span class="css"> * {<span class="attribute">color</span>: lightcoral}</span></span><br><span class="line"><span class="css"> <span class="selector-tag">h1</span><span class="selector-class">.title</span> {<span class="attribute">color</span>: black}</span></span><br><span class="line"><span class="undefined"></span><span class="tag"></<span class="name">style</span>></span></span><br><span class="line"><span class="tag"><<span class="name">body</span>></span></span><br><span class="line"> <span class="tag"><<span class="name">h1</span> <span class="attr">class</span>=<span class="string">"title"</span>></span>Meerkat <span class="tag"><<span class="name">em</span>></span>Central<span class="tag"></<span class="name">em</span>></span><span class="tag"></<span class="name">h1</span>></span></span><br><span class="line"> <span class="tag"><<span class="name">p</span>></span>Welcome to the best place on the web...<span class="tag"></<span class="name">p</span>></span></span><br><span class="line"><span class="tag"></<span class="name">body</span>></span></span><br></pre></td></tr></table></figure><p></p><p>显示如下图所示：</p><p><img src="img/15715845-file_1491625597713_fdef.png" alt="15715845-file_1491625597713_fdef.png"></p><p>结果证明了0特殊性比无特殊性要强。</p><h2 id="按权重和来源排序"><a href="#按权重和来源排序" class="headerlink" title="按权重和来源排序"></a>按权重和来源排序</h2><p>一般来说，权重由大到小的顺序为：</p><p>1.读者的重要声明。</p><p>2.创作人员的重要声明。</p><p>3.创作人员的正常声明。</p><p>4.读者的正常声明。</p><p>5.用户代理声明。</p><p>CSS中的样式一共有三种来源：创作人员、读者和用户代理。</p><p>创作人员（author’s+style）样式应该是我们最熟悉的，如果你是一个前端开发者，那么你写的那些样式就叫做创作人员样式。</p><p>用户代理样式（agent’s+style）。用户代理也就是我们通常所说的浏览器（IE、Firefox等等），这些浏览器会提供一些默认的样式。</p><p>读者样式（reader’s+style）。所谓读者自然就是浏览网页的用户，有些时候这些用户里可能会有人不满意网页的配色，或者字体大小，这时候他们就是通过浏览器提供的接口为网站添加读者样式。</p><p>比如：</p><figure class="highlight css"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br></pre></td><td class="code"><pre><span class="line"><span class="selector-tag">p</span> <span class="selector-tag">em</span> {<span class="attribute">color</span>: black;} <span class="comment">/* author's style sheet*/</span> </span><br><span class="line"><span class="selector-tag">p</span> <span class="selector-tag">em</span> {<span class="attribute">color</span>: yellow;} <span class="comment">/* reader's style sheet*/</span></span><br></pre></td></tr></table></figure><p></p><p>第一条规则将胜出。</p><figure class="highlight css"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br></pre></td><td class="code"><pre><span class="line"><span class="selector-tag">p</span> <span class="selector-tag">em</span> {<span class="attribute">color</span>: black <span class="meta">!important</span>;} <span class="comment">/* author's style sheet*/</span> </span><br><span class="line"><span class="selector-tag">p</span> <span class="selector-tag">em</span> {<span class="attribute">color</span>: yellow <span class="meta">!important</span>;} <span class="comment">/* reader's style sheet*/</span></span><br></pre></td></tr></table></figure><p></p><p>第二条规则将胜出。</p><script>$(".post-body a").not(".thispage").addClass("ignore-href").attr("target","_blank")</script></div><div></div><div><div style="padding:10px 0;margin:20px auto;width:90%;text-align:center;color:#878787" 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