CSS is hard to maintain and scale without a well-defined approach. Here are five CSS development methodologies and style guides that can help.
SMACSS stands for Scalable and Modular Architecture for CSS. A core idea behind this CSS development approach is minimizing the depth of selectors in order to keep them modular as well as to lower dependency on the HTML structure. In SMACSS, there are five style-rule categories: Base, Layout, Module, State, and Theme.
CSS Guidelines is a comprehensive guide for writing maintainable CSS. It has rules such as limiting stylesheet lines to 80 characters, using soft tabs equal to four spaces, and so forth.
BEM stands for Block, Element, Modifier. In this front-end development methodology, a block is a discrete component, or a "building block". An element is a part of a block; for instance, a search form block has two elements: (1) a text input box and (2) a button. A modifier is a property that changes a block or element visually or behaviorally. Each block has a unique name (a CSS class).
Principles of writing consistent, idiomatic CSS — or idiomatic CSS for short — is a CSS development style guide. It includes guidelines for code-formatting, commenting in the stylesheet, and such. The core principle behind this style guide is to keep your source CSS consistent and readable.