css 属性 border-top-width 是用于设置盒模型的上边框的宽度

{{InteractiveExample("CSS Demo: border-top-width")}}

```css interactive-example-choice border-top-width: thick;

```css interactive-example-choice
border-top-width: 2em;

```css interactive-example-choice border-top-width: 4px;

```css interactive-example-choice
border-top-width: 2ex;

```css interactive-example-choice border-top-width: 0;

```html interactive-example
<section class="default-example" id="default-example">
  <div class="transition-all" id="example-element">
    This is a box with a border around it.
  </div>
</section>

```css interactive-example

example-element {

background-color: palegreen; color: #000; border: 0 solid crimson; padding: 0.75em; width: 80%; height: 100px; }

## Syntax

```css
/* 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;

Values

• <br-width>

  • : 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：

  thin		A thin border
  medium		A medium border
  thick		A thick border

  [!NOTE] 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 thin ≤ medium ≤ thick, and the values are constant within a single document.

Formal syntax

{{csssyntax}}

Example

HTML

<div>Element 1</div>
<div>Element 2</div>

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;
}

Result

{{EmbedLiveSample('Example', '100%')}}

Specifications

{{cssinfo}}

Browser compatibility

See also

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