I think it's actually saying a#foo is worse than #foo (assuming you're not reusing #foo in which case, conceptually, shouldn't it be a.foo?). Indeed, I'd argue

I think it's actually saying a#foo is worse than #foo (assuming you're not reusing #foo in which case, conceptually, shouldn't it be a.foo?). Indeed, I'd argue it ought to say that you EITHER ought to use a.foo or #foo depending on what you're doing in the page. I'm not an efficiency nut, but it's quite possible #foo runs faster than .foo and this might be important to you, even if giving multiple things the same ID makes you feel dirty.