Why Your z-index: 9999 Isn't Working (How to Use z-index in CSS) 

Is your z-index not working? Are you confused why sometimes, no matter what you do, you can't place one element on top of the other? Let me explain the rules of z index in this video, what the stacking context is, and never be confused about the weird rules of z-index in CSS again...
More about stacking context: developer.mozilla.org/en-US/docs/Web/CSS/CSS_Positioning/Understanding_z_index/The_stacking_context

Наука