Is a <length> value giving a hint of the optimal width of the column. The actual column width may be wider (to fill the available space), or narrower (only if the available space is smaller than the specified column width). The length must be strictly positive or the declaration is invalid.
Is a strictly positive <integer> describing the ideal number of columns into which the content of the element will be flowed. If the column-width is also set to a non-auto value, it merely indicates the maximum allowed number of columns.