Detailed answer
Strong indicates importance or urgency; em indicates stress emphasis.
Their default bold and italic styles are presentation conventions, not the reason to choose them. If you only want a visual treatment, use CSS or another suitable element. Consider how the emphasis changes the actual meaning of the sentence.
Example
<p><strong>Save your work</strong> before closing.</p>
<p>I said <em>one</em> file.</p>
Understanding the behavior
Strong marks importance, seriousness or urgency; em marks stress emphasis that can change the meaning of a sentence. Bold and italic appearance alone do not determine the right element. CSS can reproduce either appearance without adding those meanings.
Practical implementation notes
Read the sentence aloud with emphasis in different positions. If the styling is purely decorative or follows a publication convention, consider CSS or another more appropriate text element.
How to check it
Read the sentence aloud and decide which words express importance versus stress.
Expected result
The chosen element matches the intended meaning rather than only the desired font style.
Watch out: Do not choose em merely to get italic lettering.
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