Too Long; Didn't Read
Modular CSS is the philosophy or methodology or an approach of breaking down webpage styling into smaller independent components that are consistent, reusable and hold on to its visual integrity in any part of the webpage. The core concept present throughout the various guidelines or methodologies is how to identify the smaller parts/components that make up a webpage and then how to construct them using a consistent naming pattern. OOCSS (OOCSS) closely follows the concept of object-oriented programming principle of separation of concern.