<em>
Stress emphasis that changes the meaning of the sentence.
Baseline: Widely available · Inline text semantics
When to use it
Where a speaker would stress the word: 'I <em>never</em> said that' differs from 'I never said <em>that</em>'.
When not to
Not for italics-as-style (that's i or CSS) and not for importance (that's strong).
Example
Do not add more flour — the dough is supposed to be sticky.
<p>Do <em>not</em> add more flour — the dough is supposed to be sticky.</p>