border-top-width - CSS（层叠样式表） 编辑

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

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Syntax

/* 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

<line-width>
where 
<line-width> = <length> | thin | medium | thick

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

Specifications

Specification	Status	Comment
CSS Backgrounds and Borders Module Level 3 border-top-width	Candidate Recommendation	No significant change.
CSS Level 2 (Revision 1) border-top-width	Recommendation	No significant change.
CSS Level 1 border-top-width	Recommendation	Initial definition.

初始值	medium
适用元素	all elements. It also applies to ::first-letter.
是否是继承属性	否
计算值	the absolute length or 0 if border-top-style is none or hidden
Animation type	a length

Browser compatibility

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See also

• The other border-width-related CSS properties: border-left-width, border-right-width, border-bottom-width, and border-width.
• The other border-top-related CSS properties: border, border-top, border-top-style, and border-top-color.