In CSS, the fundamental visual rendering model places all components of the
document tree in physical and virtual rectangular boxes, each having a specific
height and width. The dimension properties describe the details influencing
the calculation and visual rendering of an element's line, in-line and block box
dimensions. The actual values for an element's visually rendered height and
width are dependent on many criteria, including the intrinsic type of the current
element and the dimensions of the element's containing block.
Official Reference: CSS Level 1, Sections 5.4.8 and 5.5.23-24