<mark>

Web authors have used the elements <em> and <strong> to give emphasis or importance to short passages of text: to stress a word in a sentence so that meaning is clear or add weight to a warning message. These elements have been used to highlite important passages, too; now, <mark> provides a significant way to create a highlite.
now, &#60;mark&#62; provides <mark>a significant way to create a highlite</mark>.

In cases that I have seen, <mark> doesn't impart any visual change to the enclosed text. The <mark> element provides a semantic tag that can be styled with css to create a visual highlite.
The &#60;mark&#62; element provides <mark style="background-color:#EEB; padding:1px">a semantic tag that can be styled with css to create a visual highlite</mark>.