Inline-level elements are those that generate an element box within a line of text and do not break up the flow of that line. The best-known inline element is the a element in HTML and XHTML. Other examples are span and em. These elements do not generate a break before or after themselves, so they can appear within the content of another element without disrupting its display.

Note that although the CSS block and inline elements have a great deal in common with HTML and XHTML block- and inline-level elements, there is an important difference. In HTML and XHTML, block-level elements cannot descend from inline-level elements, whereas in CSS, there is no restriction on how display roles can be nested within each other.