I'm asking because I remember a time when it wasn't possible
to create a div box on another one, and I was wondering when was it introduced.
It's probably a matter of CSS version, isn't it?
The z-index CSS property, which introduced the 'z-axis' and the ability to stack elements, was introduced with CSS2.
CSS2 was published by the W3C as a Recommendation in May 1998. So you're right -- it wasn't always possible, but it's been around for a while!
Layers were first possible in Netscape 3 using the
<layer> tag. This tag was proprietary to Netscape and was not supported by Internet Explorer. Support for it was abandoned in Netscape 6.
IE 4 used
<div> tags and the
z-index CSS property to create layers. That has since become standard.