About
em is an HTML element that set a strong emphasis on words of a sentence.
If you want to add an extra comment (extra voice) on the actual text, you may want to use the idiomatic element instead.
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Example
The default font styling is italic and should have been bold instead to make a strong difference with the the idiomatic element
Word
This is <em>not</em> what I intended to do
Sentence
<p><em>Cats are cute animals!</em></p>