Web Design in a Nutshell, 3rd Edition

The vertical-align property applies to inline text elements as well as nontext elements that may appear in the flow of text, such as images or form inputs. Images and inputs are examples of replaced elements, because the source document contains only a reference to the element that is replaced by the actual content when the page is assembled. Most XHTML elements are non-replaced elements, which means their content appears in the source document, like the text of a paragraph (p).