From Xojo Documentation

Property (As Integer )
aWebControl.Zindex = newIntegerValue
IntegerValue = aWebControl.Zindex

New in 2010r4

Supported for all project types and targets.

The layer of the control. By default, the backmost control will have a value of zero. The higher the value, the closer the control is to the front. At runtime, you can set this value to anything you want but the lowest value will always be the backmost control.