<div class="hoge">
  <p>ウィンドウ横幅の50%でwidth指定</p>
  <div class="hoge__fuga">
     <p>ウィンドウ横幅の25%でwidth指定</p>
  </div>
</div>
.hoge {
  width: 50vw;
  background: #ccc;
  &__fuga {
    width: 25vw;
    background: #aaa;
  }
}
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