Web developpers used to use the z-index attribut with a value of 9999 to be sure that the given block is above all others. However, it's using z-index attribute the wrong way. Indeed, just with HTML flow and opacity management, you should be able to manage the use case.
- English version : What no one told you about z-index
- French version : Comprendre z-index et les ordres d'empilement
Happy reading !