On Windows Phone 8.1 the camera image is shown through <video> tag with a z-order style attribute rather than z-index. Same thing for the Cancel <button> tag.
As far as I know, z-order is the concept of layers in an web page but the CSS property managing it is z-index. The z-index CSS property specifies the z-order of an element and its descendants.
I have a issue because Bootstrap navbar has a z-index of 1030 and the <video> tag for camera has a z-order of 999 so it isn't fullscreen. When I lower navbar z-index, it doesn't have any influence on the layers because Internet Explorer seems to understand z-order as a priority. If z-index was used for <video> and <button> tag, everything could be easily solved throught CSS.