 <p>In addition to Boltclock's answer, there's a difference in how css walks through the DOM, which can have an effect on the processing speed of the stylesheet. For example, Mozilla advises not to use the element before the class: <a href="https://developer.mozilla.org/en/Writing_Efficient_CSS" rel="nofollow noreferrer">https://developer.mozilla.org/en/Writing_Efficient_CSS</a></p> <p>In case of id's I tend to use it anyway though, purely for readability reasons.</p>