In this article Max explains how to build a highlighter marker effect using only CSS.
The result is a nice looking effect to spice up the default highlighting style of the <mark> element.
This is the resulting CSS code from the article (now integrated in the blog here):

mark {
  margin: 0 -0.4em;
  padding: 0.1em 0.4em;
  border-radius: 0.8em 0.3em;
  background: transparent;
  background-image: linear-gradient(
    to right,
    rgba(255, 225, 0, 0.1),
    rgba(255, 225, 0, 0.7) 4%,
    rgba(255, 225, 0, 0.3)
  );
  -webkit-box-decoration-break: clone;
  box-decoration-break: clone;
}