AngleSharp by Florian Rappl

CssZIndexProperty

sealed class CssZIndexProperty : CssProperty, ICssZIndexProperty, ICssProperty

Information can be found on MDN: https://developer.mozilla.org/en-US/docs/Web/CSS/z-index

public int? Index { get; }

Gets the index in the stacking order, if any.

public void SetIndex(int? index)