stylelint — A mighty, modern CSS linter that helps you enforce consistent conventions and avoid errors in your stylesheets. I'm a core team member.
stylelint-order — A plugin pack of order related linting rules for stylelint.
PostCSS Sorting — PostCSS plugin to keep rules and at-rules content in order.
Stylelint: The Style Sheet Linter We’ve Always Wanted on Smashing Magazine.
Working with Images in Stylesheets with PostCSS on CSS-Tricks.