jftr, <span> tags were devised with a 'structural' effect in mind, but will also support the 'presentational' effect you want.
Seeing as you are creating a presentational emphasis rather than a structural one, it is recommended that you use a semantic, presentational tag (i.e. <em>) as these tags can then be used to emphasise that text in alternative web reading/browing software (i.e. Lynx, Jaws, ...).
Or to put it another way...
ditto.
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That'll learn me to have breakfast mid-post
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