Frontend work is usually a rather messy job if things are not organized properly from the start. A CSS styleguide is a useful tool for establishing common criteria for the team, in terms of both CSS coding syntax and CSS architecture.
The CSS styleguide we created is one that I use for all frontend projects. It establishes an unambigous way of writing CSS selectors and declarations taking into account higher level tools like LESS or SCSS which are used in most of nowadays projects.
Another aspect that’s worth to be mentioned about this styleguide is that it specifies the patterns to be applied when using BEM architecture for building component-based UIs. This BEM approach for building scalable web UIs has gained a lot of popularity lately and has been adopted by most major frontend experts.