答：对边框使用透明颜色
transparent对于没有CSSwidth和的元素的边框，您可以使用和纯色的组合轻松地创建指向上，下，左或右方向的三角形或插入符号图标height。让我们看一个例子来了解它是如何工作的：
例试试这个代码»
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="utf-8">
<title>Making Directional Triangle or Caret Icons with CSS</title>
<style>
.caret {
width: 0;
height: 0;
display: inline-block;
border: 10px solid transparent;
}
.caret.down{
border-top-color: black;
}
.caret.right{
border-left-color: black;
}
.caret.up{
border-bottom-color: black;
}
.caret.left{
border-right-color: black;
}
</style>
</head>
<body>
<h3>Triangle or Caret Icon Pointing Down</h3>
<div><span class="caret down"></span></div>
<hr>
<h3>Triangle or Caret Icon Pointing Right</h3>
<div><span class="caret right"></span></div>
<hr>
<h3>Triangle or Caret Icon Pointing Up</h3>
<div><span class="caret up"></span></div>
<hr>
<h3>Triangle or Caret Icon Pointing Left</h3>
<div><span class="caret left"></span></div>
</body>
</html>
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