[dspillett]

But they do exist, as do other sub-pixel layouts (pentile, RGBG, ...). If you use sub-pixel rendering assuming RGB you are going to look worse on some devices (many Samsung phones, IIRC).

Also, if you use sub-pixel tricks in static resources such as images (or your text renderer is not aware that the sub-pixels may not always be arranged horizontally) you are going to have colour halo effects when a device is rotated. Windows is doing that (optimising for horizontal RGB, when vertical RGB is the reality) right now on the screen I have rotated to portrait aspect. It isn't stark enough that I can see any colour halo at this dot pitch, in fact the effect is very subtle, but seen side-by-side text looks a little blury on that screen compared to the other screen (practically the same dot pitch, but landscape so actually is a horizontal RGB layout). Someone with better eyes (mine are somewhat shite) might find it more irritating that I do. I must get around to telling Windows to just use greyscale text trickery...