<em>: The Emphasis element

The HTML <em> element marks text that has stress emphasis. The <em> element can be nested, with each level of nesting indicating a greater degree of emphasis.

This element only includes the global attributes.

The <em> element is for words that have a stressed emphasis compared to surrounding text, which is often limited to a word or words of a sentence and affects the meaning of the sentence itself.

Typically this element is displayed in italic type. However, it should not be used to apply italic styling; use the CSS font-style property for that purpose. Use the <cite> element to mark the title of a work (book, play, song, etc.). Use the <i> element to mark text that is in an alternate tone or mood, which covers many common situations for italics such as scientific names or words in other languages. Use the <strong> element to mark text that has greater importance than surrounding text.

New developers are often confused at seeing multiple elements that produce similar results. <em> and <i> are a common example, since they both italicize text. What's the difference? Which should you use?

By default, the visual result is the same. However, the semantic meaning is different. The <em> element represents stress emphasis of its contents, while the <i> element represents text that is set off from the normal prose, such a foreign word, fictional character thoughts, or when the text refers to the definition of a word instead of representing its semantic meaning. (The title of a work, such as the name of a book or movie, should use <cite>.)

This means the right one to use depends on the situation. Neither is for purely decorative purposes, that's what CSS styling is for.

An example for <em> could be: "Just do it already!", or: "We had to do something about it". A person or software reading the text would pronounce the words in italics with an emphasis, using verbal stress.

An example for <i> could be: "The Queen Mary sailed last night". Here, there is no added emphasis or importance on the word "Queen Mary". It is merely indicated that the object in question is not a queen named Mary, but a ship named Queen Mary. Another example for <i> could be: "The word the is an article".

The <em> element is often used to indicate an implicit or explicit contrast.

<p>
  In HTML 5, what was previously called
  <em>block-level</em> content is now called
  <em>flow</em> content.
</p>