SOURCE

<div class="box ez-horizontal-layout">
  <dl class="item-left ez-horizontal-layout__left"></dl>
  <dl class="item-right ez-horizontal-layout__right"></dl>
</div>

dl{ margin: 0; }
.box{
  .item-left, .item-right{ height: 100px; }
  .item-left{ background: gray; }
  .item-right{ background: #FFF; }
}

@mixin ez-horizontal-layout($--item-left-width: 100px) {
  .ez-horizontal-layout{
    &{
      position: relative;
    }

    &__left {
      width: $--item-left-width;
    }

    &__right {
      position: absolute;
      top: 0;right: 0;bottom: 0;
      left: $--item-left-width
    }
  }
}

@include ez-horizontal-layout(100px);