1. 画对角线

使用线性渐变, 因为是正方形, 所以是45度角

.block {
    background: linear-gradient(45deg, transparent 49.5%, #fff000 50.5%, #fff000 50.5%, transparent 50.5%);
}
  1. 代码链接

https://codepen.io/klren0312/full/zYqPVXZ

表格对角线

<style>
  .line {
  padding: 20px 0;
  position: relative;
  background: linear-gradient(11deg, transparent 48.5%, #ff0000 49.5%, #ff0000 36.5%, transparent 51.5%);
.top,
.bottom{
  position: absolute;
.top {
  top: 0;
  right: 20px;
.bottom {
  bottom: 0;
  left: 20px;
</style>