使用CSS创建div的对角顶部

<div id="diagonal-top"></div>

#diagonal-top {
  position: relative;
  width: 200px;
  height: 100px;
  background-color: #f0f0f0;
}

#diagonal-top:before {
  content: "";
  position: absolute;
  top: -50px;
  left: 0;
  width: 0;
  height: 0;
  border-left: 100px solid transparent;
  border-right: 100px solid transparent;
  border-bottom: 50px solid #f0f0f0;
}

<link rel="stylesheet" href="styles.css">