Dear all:
I have a header with 60% width occupied by a video, to the right of which is an aside section taking the remaining 40%. The aside section has three items fixed horizontally on top of each other. So the header has four elements in total. To account for possible overlaps between the four elements, I declared a z-index of -1 to the top right element, with the rest elements getting z-index 1 to 3. Then the element with z-index -1 disappeared. I then gave it z-index 1 and gave the others 2-4. Why has z-index -1 caused the element to disappear?