<mark>
Relevance marked by the reader, not emphasis from the writer.
The classic case is a search result with the term highlighted. The other is quoting someone else's passage and marking the part that matters to your argument: the highlight is yours, not the original author's.
For emphasis of intonation there is <em>; for importance, <strong>. <mark> is the highlighter pen.
<blockquote>
<p>
…it is <mark>more</mark> work to build the interaction in the
page itself than to create two pages and link them.
</p>
</blockquote>