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The relation between primary and secondary is currently based on the angle between both. We might want to change it to be less strict. -primary could be the preferred orientation of that type. For example, on a phone, landscape-primary would be the landscape orientation the user prefer (for a right-handed user, likely 90 degree anti-clockwise and 90 degree clockwise for a left-handed person).
*-primary and *-secondary is now entirely up to the UA: https://dvcs.w3.org/hg/screen-orientation/rev/5397f6ae4528