A common indexing style calls for a comma and a space to be inserted between the entry and the first page number. Subsequent page numbers are separated with a comma and a space. Page numbers appear in ascending order. Often, the primary entry that has two or more secondary entries does not have any page reference itself. Definition and Usage. The z-index property specifies the stack order of an element. An element with greater stack order is always in front of an element with a lower stack order. Note: z-index only works on positioned elements (position: absolute, position: relative, position: fixed, or position: sticky). The HTML