<div class="wrap">
  <p>トップから100pxの位置より下に繰返しの背景を設定します</p>
</div>
.wrap {
  position: relative;
  background-color: #eee;
  height: 100vh;
}
.wrap::before {
  position: absolute;
  top: 100px;
  left: 0;
  width: 100%;
  height: calc(100vh - 100px);
  background-image: url("https://www.eyesofc.co.jp/webeyes/_demo/torarin/202012/bg_pattern.png");
  background-position: 0 0;
	background-repeat: repeat;
	background-size: auto;
  content: '';
}

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