CSS was not based on any specific theory, but rather on the actual rendering model of browsers at the time. This in turn was presumably based on ad-hoc decision

CSS was not based on any specific theory, but rather on the actual rendering model of browsers at the time. This in turn was presumably based on ad-hoc decisions by Netscape when they added features. For example, the float/clear model were created as a way to allow text to float around images when Netscape added support for embedded images at the dawn of time.