From SMACSS (premium bit):

>Many people would likely balk at the use of the i tag. I chose to use it because it was small, mostly devoid of semantics, and was an empty element with no content. Why is there no content? In this example, the icon supports the visible text beside it. If the icon were to sit alone, then we would add a title attribute so that it could be read by a screen reader or used as a tooltip. If you disagree and feel a span is more appropriate, I'll understand.