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---
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;
/* <length> 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><br-width></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"><div>Element 1</div>
<div>Element 2</div></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>
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