Various reasons you might want to italicize text and how to mark it up semantically.
Italicization is a concept that makes sense only in a visual context; it does not describe the content being italicized and is meaningless to visually impaired users and to other applications. Following are various types of semantic markup that will display in italics.
Emphasized text should be enclosed in the <em> element.
Text that is a heading for a section of a document should be so marked (as h2, h3, or h4) and should not be marked with any inline element.
Text that is in a language different from the dominant language of the document (i.e., the language noted in the page record) should be marked with the lang attribute.
The attribute provides the 2- or 3-letter ISO code for the language. See “Codes for the Representation of Names of Languages” for a complete list. Either the 2- or 3-letter code is acceptable.
As the French say, <span lang="fr">.
In theory, a good audio browser or screen reader will know how to correctly pronounce the enclosed material.
See also: Special characters.
A note — an explanatory note or clarification — may be enclosed in any element with class name “note”. The usual elements are p and span.
Works directly cited may be enclosed in <cite>.
Works not directly cited but merely referred to should not be semantically marked. Instead, if the title of the work would normally be italicized in print, mark it with <span class="worktitle">. These include titles of books, films, etc.