A brief detour through some docs led me to PHP’s htmlspecialchars function, where I noticed that double-quote becomes ", but apostrophe becomes '. Seemed odd, since we’re all so used to ' as the apostrophe entity. A comment on the docs claimed that there’s no such thing as ' in HTML. I was already three or four levels deep on the distraction stack, so I went and looked.
Sure enough, the HTML 4.0 spec defines 255 different character entities, and ' is not among them.
What does it mean? Nothing, really, since the browsers all understand the entity, but it demonstrates that sticking to a standard may be tougher than you think, since common practice so often exceeds what the standard guarantees.