The classification properties serve to classify elements and select which
layout models will be used in their display. In the visual media formatting
model, an element's rendering is based heavily upon, among other things,
the positioning method specified (floating, positioned or normal flow
positioning) and the classification type of the element's box (the 'display'
property.)
Related Sites
Official Reference: CSS Level 1, Sections 5.5.25-6 and 5.6.1