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'css'" rel="tag">css</a><a href="/tags/forms" class="post-tag js-gps-track" title="show questions tagged 'forms'" rel="tag">forms</a><a href="/tags/css-selectors" class="post-tag js-gps-track" title="show questions tagged 'css-selectors'" rel="tag">css-selectors</a><a href="/tags/pseudo-element" class="post-tag js-gps-track" title="show questions tagged 'pseudo-element'" rel="tag">pseudo-element</a> </div> </div> </div> </div> <span class="d-none" itemprop="commentCount">4</span> </div> </div> <div class="js-zone-container zone-container-responsive"> <div id="dfp-isb" class="everyonelovesstackoverflow everyonelovesinline-sidebar mx-auto" style="min-height: auto; height: auto; display: none;"></div> <div class="js-report-ad-button-container mx-auto" style="width: 300px"></div> </div> <div id="answers"> <a name="tab-top"></a> <div id="answers-header"> <div class="answers-subheader d-flex ai-center mb8"> <div class="flex--item fl1"> <h2 class="mb0" data-answercount=""> 3 回答 <span style="display:none;" itemprop="answerCount">3</span> </h2> </div> </div> </div> <a name="7"></a> <div id="answer-7" class="answer js-answer accepted-answer" data-answerid="7" data-parentid="4" data-score="506" data-position-on-page="1" data-highest-scored="1" data-question-has-accepted-highest-score="1" itemprop="suggestedAnswer" itemscope="" itemtype="https://schema.org/Answer"> <div class="post-layout"> <div class="votecell post-layout--left"> <div class="js-voting-container d-flex jc-center fd-column ai-stretch gs4 fc-black-200" data-post-id="7"> <button class="js-vote-up-btn flex--item s-btn s-btnunset c-pointer " data-controller="s-tooltip" data-s-tooltip-placement="right" aria-pressed="false" aria-label="Up vote" data-selected-classes="fc-theme-primary" data-unselected-classes="" aria-describedby="--stacks-s-tooltip-dgvag2l3"> <svg aria-hidden="true" class="svg-icon iconArrowUpLg" width="36" height="36" viewBox="0 0 36 36"><path d="M2 25h32L18 9 2 25Z"></path></svg> </button><div id="--stacks-s-tooltip-dgvag2l3" class="s-popover s-popovertooltip pe-none" aria-hidden="true" role="tooltip">This answer is useful<div class="s-popover--arrow"></div></div> <div class="js-vote-count flex--item d-flex fd-column ai-center fc-black-500 fs-title" itemprop="upvoteCount" data-value="103"> 103 </div> <button class="js-vote-down-btn flex--item s-btn s-btnunset c-pointer " data-controller="s-tooltip" data-s-tooltip-placement="right" aria-pressed="false" aria-label="Down vote" data-selected-classes="fc-theme-primary" data-unselected-classes="" aria-describedby="--stacks-s-tooltip-gn8ppsfv"> <svg aria-hidden="true" class="svg-icon iconArrowDownLg" width="36" height="36" viewBox="0 0 36 36"><path d="M2 11h32L18 27 2 11Z"></path></svg> </button> </div> </div> <div class="answercell post-layout--right"> <div class="s-prose js-post-body" itemprop="text"> <p><code>:before</code>and<code>:after</code>不适用于 a <code>text-area</code>（也不适用于任何不能包含另一个元素的元素，例如or <code>img</code>）<code>input</code>，因为伪元素的生成内容被放置<strong>在</strong>元素内但在该元素的内容<em>之前</em>或<em>之后</em>，并将其自身作为一个元素. 伪元素不会放置在父元素本身之前或之后（与互联网上可能找到的一些信息相反）。为了显示：</p> <p>如果你有这个 CSS：</p> <pre><code>p:before {content: 'before--'} p:after {content: '--after'} </code></pre> <p>然后像这样的html：</p> <pre><code><p>Original Content</p> </code></pre> <p><em>像源代码一样</em>有效地呈现到屏幕上：</p> <pre><code><p>before--Original Content--after</p> </code></pre> <p><strong><em>不像</em></strong> <em>源代码</em>是：</p> <pre><code>before--<p>Original Content</p>--after </code></pre> <p>这就是为什么不能包含任何<em>html</em>元素“内容”（如上面提到的那些）的标签无法识别伪元素的原因，因为没有生成该内容的“位置”。<code>textarea</code>可以包含“内容”，但只能包含纯文本内容。</p> </div> <div class="mt24"> <div class="user-action-time" style="color:#999;text-align:right;">于 2010-07-20T18:45:22.980 回答</div> </div> </div> </div> </div><a name="7"></a> <div id="answer-7" class="answer js-answer accepted-answer" data-answerid="7" data-parentid="4" data-score="506" data-position-on-page="1" data-highest-scored="1" data-question-has-accepted-highest-score="1" itemprop="suggestedAnswer" itemscope="" itemtype="https://schema.org/Answer"> <div class="post-layout"> <div class="votecell post-layout--left"> <div class="js-voting-container d-flex jc-center fd-column ai-stretch gs4 fc-black-200" data-post-id="7"> <button class="js-vote-up-btn flex--item s-btn s-btnunset c-pointer " data-controller="s-tooltip" data-s-tooltip-placement="right" aria-pressed="false" aria-label="Up vote" data-selected-classes="fc-theme-primary" data-unselected-classes="" aria-describedby="--stacks-s-tooltip-dgvag2l3"> <svg aria-hidden="true" class="svg-icon iconArrowUpLg" width="36" height="36" viewBox="0 0 36 36"><path d="M2 25h32L18 9 2 25Z"></path></svg> </button><div id="--stacks-s-tooltip-dgvag2l3" class="s-popover s-popovertooltip pe-none" aria-hidden="true" role="tooltip">This answer is useful<div class="s-popover--arrow"></div></div> <div class="js-vote-count flex--item d-flex fd-column ai-center fc-black-500 fs-title" itemprop="upvoteCount" data-value="0"> 0 </div> <button class="js-vote-down-btn flex--item s-btn s-btnunset c-pointer " data-controller="s-tooltip" data-s-tooltip-placement="right" aria-pressed="false" aria-label="Down vote" data-selected-classes="fc-theme-primary" data-unselected-classes="" aria-describedby="--stacks-s-tooltip-gn8ppsfv"> <svg aria-hidden="true" class="svg-icon iconArrowDownLg" width="36" height="36" viewBox="0 0 36 36"><path d="M2 11h32L18 27 2 11Z"></path></svg> </button> </div> </div> <div class="answercell post-layout--right"> <div class="s-prose js-post-body" itemprop="text"> <pre><code><div class='tx-div-before'></div> </code></pre> <p>在 textarea 和</p> <pre><code><div class='tx-div-after'></div> </code></pre> <p>在 textarea 之后使用此代码。并在 psedu 元素之前和之后添加。</p> </div> <div class="mt24"> <div class="user-action-time" style="color:#999;text-align:right;">于 2020-07-16T18:09:14.000 回答</div> </div> </div> </div> </div><a name="7"></a> <div id="answer-7" class="answer js-answer accepted-answer" data-answerid="7" data-parentid="4" data-score="506" data-position-on-page="1" data-highest-scored="1" data-question-has-accepted-highest-score="1" itemprop="suggestedAnswer" itemscope="" itemtype="https://schema.org/Answer"> <div class="post-layout"> <div class="votecell post-layout--left"> <div class="js-voting-container d-flex jc-center fd-column ai-stretch gs4 fc-black-200" data-post-id="7"> <button class="js-vote-up-btn flex--item s-btn s-btnunset c-pointer " data-controller="s-tooltip" data-s-tooltip-placement="right" aria-pressed="false" aria-label="Up vote" data-selected-classes="fc-theme-primary" data-unselected-classes="" aria-describedby="--stacks-s-tooltip-dgvag2l3"> <svg aria-hidden="true" class="svg-icon iconArrowUpLg" width="36" height="36" viewBox="0 0 36 36"><path d="M2 25h32L18 9 2 25Z"></path></svg> </button><div id="--stacks-s-tooltip-dgvag2l3" class="s-popover s-popovertooltip pe-none" aria-hidden="true" role="tooltip">This answer is useful<div class="s-popover--arrow"></div></div> <div class="js-vote-count flex--item d-flex fd-column ai-center fc-black-500 fs-title" itemprop="upvoteCount" data-value="-8"> -8 </div> <button class="js-vote-down-btn flex--item s-btn s-btnunset c-pointer " data-controller="s-tooltip" data-s-tooltip-placement="right" aria-pressed="false" aria-label="Down vote" data-selected-classes="fc-theme-primary" data-unselected-classes="" aria-describedby="--stacks-s-tooltip-gn8ppsfv"> <svg aria-hidden="true" class="svg-icon iconArrowDownLg" width="36" height="36" viewBox="0 0 36 36"><path d="M2 11h32L18 27 2 11Z"></path></svg> </button> </div> </div> <div class="answercell post-layout--right"> <div class="s-prose js-post-body" itemprop="text"> <p><code>:after</code>实际上，您可以在输入元素上添加内容。当元素处于活动状态时，这将添加一种提示：</p> <pre><code>#gallery_name { position:relative; } #gallery_name:focus:after { content: "Max Characters: 30"; color: #FFF; position: absolute; right: -150px; top:0px; } <input id="gallery_name" type="text" name="gallery_name" placeholder="Gallery Name"> </code></pre> </div> <div class="mt24"> <div class="user-action-time" style="color:#999;text-align:right;">于 2012-06-02T19:35:12.773 回答</div> </div> </div> </div> </div></div> </div> <div id="sidebar" class="show-votes" role="complementary" aria-label="sidebar"> <div class="module sidebar-related"> <h4 id="h-related">Related</h4> <div class="related js-gps-related-questions" data-tracker="rq=1"> <div class="spacer"> <a href="/questions/18268729" title="Question score (upvotes - downvotes)"> <div class="answer-votes large">1</div> </a> <a href="/questions/18268729" class="question-hyperlink">wpf - Wpf 从具有 DataTemplate 的 ItemsControl 触发鼠标事件（如 MouseDown、MouseUp、MouseMove、MouseLeftClick）</a> </div><div class="spacer"> <a href="/questions/18268735" title="Question score (upvotes - downvotes)"> <div class="answer-votes large">0</div> </a> <a href="/questions/18268735" class="question-hyperlink">ruby-on-rails-3 - 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