<em> marks stress emphasis. Moving it to a different word can change the meaning of a sentence. Browsers commonly render it in italics.
Smallest working example
HTML and CSS
HTML
<p>I <em>did</em> send the file.</p>
<p><em>I</em> sent the file.</p>CSS
Edit HTML or CSS to update the preview as you type. Copy code snippet copies the selected tab. Preview spacing and font defaults are not included.
Use it in a sentence
Read the examples aloud. The first stresses that the action happened; the second stresses who performed it. Nesting can increase the degree of emphasis.
Placement and accessibility
This is phrasing content, so it fits inside a paragraph or another phrasing-content container. It accepts global attributes and has no required element-specific attributes. Keep the phrase understandable without depending on a visual font change. Screen readers and user settings vary; do not assume every reader will hear a different voice.
Choose the right alternative
Use strong for importance, i for an alternate voice or a term set apart from ordinary prose, and font-style for appearance alone.
Sources and browser support
MDN: em describes the syntax and links to the specification and compatibility data. Check individual values in that table when supporting older browsers.