What is z-index?
Description: Used to set the stack hierarchy of elements, with elements with higher levels displayed first.
Note: The Z-index only works on positioned elements
Therefore, if z-index fails, it is most likely not located.
Position attribute
The default value is static, no location, normal document flow;
Relative, relative, relative, relative
Absolute: Absolute position, relative to the first parent element other than static; — Leave the document flow
Fixed: fixed position relative to the browser window. — Leave the document flow