em element represents
stress emphasis of its contents.
The level of stress that a particular piece of content has is
given by its number of ancestor
The placement of stress emphasis changes the meaning of the sentence. The element thus forms an integral part of the content. The precise way in which stress is used in this way depends on the language.
These examples show how changing the stress emphasis changes the meaning. First, a general statement of fact, with no stress:
<p>Cats are cute animals.</p>
By emphasizing the first word, the statement implies that the kind of animal under discussion is in question (maybe someone is asserting that dogs are cute):
<p><em>Cats</em> are cute animals.</p>
Moving the stress to the verb, one highlights that the truth of the entire sentence is in question (maybe someone is saying cats are not cute):
<p>Cats <em>are</em> cute animals.</p>
By moving it to the adjective, the exact nature of the cats is reasserted (maybe someone suggested cats were mean animals):
<p>Cats are <em>cute</em> animals.</p>
Similarly, if someone asserted that cats were vegetables, someone correcting this might emphasize the last word:
<p>Cats are cute <em>animals</em>.</p>
By emphasizing the entire sentence, it becomes clear that the speaker is fighting hard to get the point across. This kind of stress emphasis also typically affects the punctuation, hence the exclamation mark here.
<p><em>Cats are cute animals!</em></p>
Anger mixed with emphasizing the cuteness could lead to markup such as:
<p><em>Cats are <em>cute</em> animals!</em></p>
em element isn't a generic "italics" element.
Sometimes, text is intended to stand out from the rest of the
paragraph, as if it was in a different mood or voice. For this, the
i element is more appropriate.