As of May 2001, the W3C Standards state that <i> and <b> are deprecated.
These tags give away too much as to the formatting of the text. XML and XHTML are meant to describe the data, not the layout. To that end XHTML v1.0 states that you must use the following instead:
<em>Emphasis (usually italicised)</em>
<strong>Strong emphahsis (usually bolded)</strong>