Avoid using the following HTML elements, and use the appropriate CSS instead.

• b

• i

• tt

• big

• small

• strike

• s

• u

• font

• basefont

These elements are not prohibited, but it is recommended to either use other semantic elements or use CSS when feasible. Refer to discussion at Strongly emphasizing semantics.

When to use this technique
Use this technique when the majority of the audience will access your content via the latest versions of user Agents.
In 2005 these and later versions of user agents support CSS sufficiently to use this technique. (note - I did not verify these actual version numbers - this is for discussion purposes only!)
Firefox 1.0
IE 5.0
Mozilla 1.4
Netscape 6
Opera 6
Safari ?

Note - I'm not quite sure how to deal with assistive technologies in this example.

More Information

Most modern day user agents support at least CSS 1.0.  Note that even though the browser may support CSS, the user often can disable CSS.   Note that if CSS is turned off, any elements that have been styled using CSS will appear without any presentational information as text in the user agent default font.  This is not necessarily an accessibility problem but something you should consider when authoring your content.  If the accessibility and readability of your content depends upon the actually styling of text, you may consider using the presentational HTML elements rather than the CSS advocated in this technique.
However, if the majority of your audience will be using recent versions of user agents, styling using CSS is the preferred method as it separates the presentation from the actual markup.  This gives the user the option to turn off the presentational aspects of the content if desired by turning off CSS in the user agent..  Another benefit of using CSS is the user may override certain aspects of the styles via a personal style sheet.  A benefit to the content author is more fine grained control of the content presentation via CSS.