The HTML5 specification introduced an interesting new tag: <mark>. You can use <mark> to highlight parts of your text.

A first question is how it compares to tags like <strong> and <em>. I found a good answer on Stack Overflow. In summary, <strong> and <em> represent pieces of text that are relevant within the context of the page. On the other hand, the <mark> element represents a run of text that is highlighted due to its relevance in another context.

Upon reflection, I believe that typical surfing behavior is a good application of <mark>. Typically, people land on a page "out of context". They will first glance over it, looking for key phrases to decide whether the content is interesting for them. Using <mark> to highlight key phrases may help a reader to decide whether it is worthwhile to read the content systematically.

In Markdown, we have the option to use HTML tags like <mark> directly. However, as I was thinking about this, I realized that there is another possibility. We can support lightweight markup for this tag by taking advantage of a redundancy in Markdown. In standard Markdown, you can either use asterisks (*) or underscores (_) to indicate emphasis. What we could do instead is rendering the underscore (_) version using <mark>.

To experiment with this feature, I have implemented it in Urubu as a Markdown extension, and I have been using it on this page. Feedback welcome!