Google Ranking Factor – CSS
CSS (cascading style sheet) is the code that determines how elements will look once rendered in the user’s browser.
It provides information about font styles and sizes, use of colors, backgrounds, borders, margins etc CSS code can either be added directly in context in your website, written into the document head, or can be supplied to the browser via a separate file.
Usually with modern websites, there are many more CSS elements in the file than may actually be required for a given page, so CSS files can become very large. Any excessive time spent loading a CSS file will slow down the website, so optimisation of CSS will aid load speed and affects UX.