stone310
正三角border四边相等；等边三角 左右和上下比为4：3；倒三角同等边三角<!DOCTYPE html>
<html>
<head>
<meta charset="utf-8">
<style>
span{position:absolute}
.span1 {
top:50px;
border-bottom:50px solid #000;
border-left:50px solid transparent;
border-right:50px solid transparent;
border-top:50px solid transparent; /* 这条边可去掉，占位更小*/
}
.span2 {
border-bottom:40px solid #000;
border-left:30px solid transparent;
border-right:30px solid transparent;
border-top:40px solid transparent; /* 这条边可去掉，占位更小*/
}
.span3 {
border-bottom:40px solid transparent; /* 这条边可去掉，占位更小*/
border-left:30px solid transparent;
border-right:30px solid transparent;
border-top:40px solid #000;
}
</style>
</head>
<body>
<span class="span1"></span>
<span class="span2"></span>
<span class="span3"></span>
</body>
</html>