The strong and em elements

Definition

em marks stress emphasis that changes how a sentence should be understood. strong marks strong importance, seriousness, or urgency. Their default italic and bold styles are browser conventions, not their definitions.

Minimal syntax

<p><strong>Do not close this tab.</strong> The upload is still running.</p>
<p>I asked for the <em>final</em> file, not the draft.</p>

Moving the em element to another word changes the implied meaning of the second sentence. That test distinguishes real emphasis from decoration.

Choose by meaning

Use strong for a warning or a materially important phrase. Use em when vocal stress clarifies contrast. Use headings for section hierarchy, mark for relevance in a specific context, and CSS classes for visual weight that carries no extra meaning. Do not use either element merely to obtain bold or italic type.

Browser and accessibility behavior

Browsers commonly render strong in bold and em in italics. Screen readers do not all announce these semantics consistently, so critical instructions must be clear in the words themselves. The elements strengthen well-written text; they must not be the only way an urgent state or required action is conveyed.

Common failures

• An entire paragraph is wrapped in strong, eliminating any useful contrast.
• em is used for book titles or decorative italics without stress emphasis.
• A heading is replaced by bold text, removing document structure.
• Color, weight, or voice inflection is the only indication of an error.

Verification checklist

Read the sentence aloud and move the stress to another word. If the intended meaning changes, em may be correct. Remove the bold or italic CSS and confirm the instruction remains understandable. Inspect heading structure separately; importance inside a paragraph does not create a section. Test critical warnings without relying on a screen reader to announce emphasis.

Check translations and generated copy because emphasis can move when sentence structure changes. Apply the element to the words whose stress or importance is part of the meaning, then confirm the sentence remains clear when the visual style is unavailable.