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The border-top-width CSS property sets the width of the top border of a box.


/* 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;


Defines the width of the border, either as an explicit nonnegative <length> or a keyword. If it's a keyword, it must be one of the following values:
A thin border
A medium border
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


<br-width> = <length> | thin | medium | thick



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


div { 
  border: 1px solid red;
  margin: 1em 0;

div:nth-child(1) {
  border-top-width: thick;
div:nth-child(2) {
  border-top-width: 2em;



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

Initial valuemedium
Applies toall elements. It also applies to ::first-letter.
Computed valuethe absolute length or 0 if border-top-style is none or hidden
Animation typea length
Canonical orderthe unique non-ambiguous order defined by the formal grammar

Browser compatibility

FeatureChromeEdgeFirefoxInternet ExplorerOperaSafari
Basic support112143.51
FeatureAndroid webviewChrome for AndroidEdge mobileFirefox for AndroidOpera AndroidiOS SafariSamsung Internet
Basic support2.3 ? Yes4111 ?

See also

Document Tags and Contributors

Last updated by: wbamberg,