A function that takes content (HTML/JS/PHP/etc) and CSS, and returns only the used CSS. PurifyCSS does not modify the original CSS files. You can write to a new file, like minification. If your application is using a CSS framework, this is especially useful as many selectors are often unused. Array of glob file patterns to the files to search through for used classes (HTML, JS, PHP, ERB, Templates, anything that uses CSS selectors).