I don't know if I am sending suggestion this to the right address, but it is the only suitable one I can find.
I have searched for although have been unable to find any referrence at the W3C website to datum as a valid css feature, and so I presume that no such feature exists.
Datum would ideally enable website developers to set the zero point from which all other measurements are made. For example if the datum were set to datum: width="50%;" then all other horizontal positioning measurements would be made from that datum including negative measurements such as position:absolute; left:-200px, which would position an element 200 pixels to the left of the datum, while position:absolute; left:200px would position the element 200 pixels to the right of the datum. This already works, although the item positioned using the negative measurement is located the left of the browser window.
I believe that such a feature would be of great value to web developers in creating websites that are always centered within the browser window.