The nuances of using a negative z-index value in CSS

If z-index is not specified for an element, its value should be interpreted as 0. This means that an element with a z-index value of -1 will be below an element without z-index.

We can rework the example with our two elements like this: just set the second element's z-index to -1, and it will be placed under the first element:

#elem1 {
	/* don't set z-index */
}
#elem2 {
	z-index: -1;
}

Result of code execution:

Repeat the page according to this example: