Styling with CSS covers a broad range of topics so for more of a deep-dive check outMozilla's CSS Guide. Creating a style guide, some say, is a necessity when developing any kind of project. For example, Wikipedia defines it as : “A style guide (or manual of style) is a set of standards for the writing and design of documents, either for general use or for a specific publication, organization, or field. A style guide establishes and enforces style to improve communication. To do that, it ensures consistency within a document and across multiple documents and enforces best practice in usage and in language composition, visual composition, orthography and typography.” .
Creating a set of CSS conventions — often called a CSS style guide — can streamline your web development workflow. It’s useful for large teams and solo developers alike. CSS style guides are a group of documents that outline the coding style, best practices, and visual design properties (e.g. colors, layout grid dimensions, etc.) of a site. The goal of having a style guide is to maintain consistency across a product. If you’re in the process of creating a CSS style guide, or if you want to learn about the CSS practices that large-scale websites use, check out the following list of style guides.
I've gathered a list of UI designers and developer's favorite styleguides. Keep in mind that a styleguide is defined as a set of standards and rules on how to use and write CSS code. It often contains global branding definitions like colors and typography and a set of reusable components for building a more consistent and maintainable project.