I think this all makes sense to me now - you introducing me to the > made it all slide neatly into place.

It seems to be something to do with pseudo-classes having their precedence set on a different scale from that of the rest of the stylesheet. After all, if, as part of this same page, I decided to fiddle with the visited link styles, even though that would only be one term (a:visited), I'd expect that to apply to all links, even those that have also been defined with greater specificity elsewhere. This also explains why order of appearance matters with pseudo-classes when it doesn't with anything else.

Of course, having said it makes sense to me now, some know-it-all is going to pop up and explain the crucial thing that I've missed which blows the whole thing apart <looks suspiciously at onebyone>.