<h1>
  Hello <span>world</span>
</h1>
h1 {
  span {
    position: relative;
    font-weight: normal;
    &:after {
      content: '';
      position: absolute;
      bottom: 2px;
      left: -4px;
      right: -4px;
      height: 9px;
      background: skyblue;
      z-index: -1;
    }
  }
}
View Compiled

External CSS

This Pen doesn't use any external CSS resources.

External JavaScript

This Pen doesn't use any external JavaScript resources.