---
title: border-top-width
slug: Web/CSS/border-top-width
translation_of: Web/CSS/border-top-width
---
<div>{{CSSRef}}</div>

<p>css属性 <strong><code>border-top-width</code></strong> 是用于设置盒模型的上边框的宽度</p>

<div>{{EmbedInteractiveExample("pages/css/border-top-width.html")}}</div>



<h2 id="Syntax" name="Syntax">Syntax</h2>

<pre class="brush:css no-line-numbers">/* Keyword values */
border-top-width: thin;
border-top-width: medium;
border-top-width: thick;

/* &lt;length&gt; values */
border-top-width: 10em;
border-top-width: 3vmax;
border-top-width: 6px;

/* Global keywords */
border-top-width: inherit;
border-top-width: initial;
border-top-width: unset;
</pre>

<h3 id="Values">Values</h3>

<dl>
 <dt><code>&lt;br-width&gt;</code></dt>
 <dd>上边框的宽度属性的有效值要么是一个具体的非负数，要么是一个关键字。如果是关键字，必须是一下几种：
 <table class="standard-table">
  <tbody>
   <tr>
    <td style="vertical-align: middle;"><code>thin</code></td>
    <td style="vertical-align: middle;">
     <div style="margin: 0.5em; width: 3em; height: 3em; border-top-style: solid; border-top-width: thin; background-color: palegreen;"> </div>
    </td>
    <td style="vertical-align: middle;">A thin border</td>
   </tr>
   <tr>
    <td style="vertical-align: middle;"><code>medium</code></td>
    <td style="vertical-align: middle;">
     <div style="margin: 0.5em; width: 3em; height: 3em; border-top-style: solid; border-top-width: medium; background-color: palegreen;"> </div>
    </td>
    <td style="vertical-align: middle;">A medium border</td>
   </tr>
   <tr>
    <td style="vertical-align: middle;"><code>thick</code></td>
    <td style="vertical-align: middle;">
     <div style="margin: 0.5em; width: 3em; height: 3em; border-top-style: solid; border-top-width: thick; background-color: palegreen;"> </div>
    </td>
    <td style="vertical-align: middle;">A thick border</td>
   </tr>
  </tbody>
 </table>

 <div class="note">
 <p><strong>Note:</strong> Because the specification doesn't define the exact thickness denoted by each keyword, the precise result when using one of them is implementation-specific. Nevertheless, they always follow the pattern <code>thin ≤ medium ≤ thick</code>, and the values are constant within a single document.</p>
 </div>
 </dd>
</dl>

<h3 id="Formal_syntax">Formal syntax</h3>

{{csssyntax}}

<h2 id="Example">Example</h2>

<h3 id="HTML">HTML</h3>

<pre class="brush: html">&lt;div&gt;Element 1&lt;/div&gt;
&lt;div&gt;Element 2&lt;/div&gt;</pre>

<h3 id="CSS">CSS</h3>

<pre class="brush: css">div {
  border: 1px solid red;
  margin: 1em 0;
}

div:nth-child(1) {
  border-top-width: thick;
}
div:nth-child(2) {
  border-top-width: 2em;
}</pre>

<h3 id="Result">Result</h3>

<p>{{EmbedLiveSample('Example', '100%')}}</p>

<h2 id="Specifications">Specifications</h2>

<table class="standard-table">
 <thead>
  <tr>
   <th scope="col">Specification</th>
   <th scope="col">Status</th>
   <th scope="col">Comment</th>
  </tr>
 </thead>
 <tbody>
  <tr>
   <td>{{SpecName('CSS3 Backgrounds', '#the-border-width', 'border-top-width')}}</td>
   <td>{{Spec2('CSS3 Backgrounds')}}</td>
   <td>No significant change.</td>
  </tr>
  <tr>
   <td>{{SpecName('CSS2.1', 'box.html#border-width-properties', 'border-top-width')}}</td>
   <td>{{Spec2('CSS2.1')}}</td>
   <td>No significant change.</td>
  </tr>
  <tr>
   <td>{{SpecName('CSS1', '#border-left-width', 'border-top-width')}}</td>
   <td>{{Spec2('CSS1')}}</td>
   <td>Initial definition.</td>
  </tr>
 </tbody>
</table>

<p>{{cssinfo}}</p>

<h2 id="Browser_compatibility" name="Browser_compatibility">Browser compatibility</h2>



<p>{{Compat("css.properties.border-top-width")}}</p>

<h2 id="See_also" name="See_also">See also</h2>

<ul>
 <li>The other border-width-related CSS properties: {{Cssxref("border-left-width")}}, {{Cssxref("border-right-width")}}, {{Cssxref("border-bottom-width")}}, and {{Cssxref("border-width")}}.</li>
 <li>The other border-top-related CSS properties: {{Cssxref("border")}}, {{Cssxref("border-top")}}, {{Cssxref("border-top-style")}}, and {{Cssxref("border-top-color")}}.</li>
</ul>