<!DOCTYPE html>
<html lang="ja">
<head>
<meta charset="UTF-8">
<meta name="viewport" content="width=device-width, initial-scale=1">
<style>/* 斜め背景の下線 */
.under-naname-basic span {
  display: inline-block;
  line-height: 1.6;
  min-height: 1em;
  position: relative;
  z-index: 10;
}

.under-naname-basic span::after {
  content: "";
  display: inline-block;
  position: absolute;
  bottom: 0;
  left: 0;
  width: 100%;
  height: 6px;
  background: linear-gradient(-45deg, #ff1481 0%, #ff1481 25%, #fff 25%, #fff 50%, #ff1481 50%, #ff1481 75%, #fff 75%, #fff 100%)repeat fixed;
  background-size: 10px 10px;
  z-index: -1;
}</style>
</head>
<body>
<div>
<p class="under-naname-basic">テキストに<span>斜め背景</span>の下線</p>
</div>
<script>
</script>
</body>
</html>
/* 斜め背景の下線 */
.under-naname-basic span {
  display: inline-block;
  line-height: 1.6;
  min-height: 1em;
  position: relative;
  z-index: 10;
}

.under-naname-basic span::after {
  content: "";
  display: inline-block;
  position: absolute;
  bottom: 0;
  left: 0;
  width: 100%;
  height: 6px;
  background: linear-gradient(-45deg, #ff1481 0%, #ff1481 25%, #fff 25%, #fff 50%, #ff1481 50%, #ff1481 75%, #fff 75%, #fff 100%)repeat fixed;
  background-size: 10px 10px;
  z-index: -1;
}