"Because a css request blocks the render of the screen until it downloads. Browsers have a lookahead parser which continues to parse the page and fetch other resources to speed the rendering and load page time so you can get away with having resources not in the head, or even downloaded asynchronously (using a tiny piece of JS) The guidance about critical render path and css is the best place to start https://developers.google.com/web... and I encourage you to read it to learn about what happens in the browser and why the guidance exists. The guidance that psi gives is designed to stop you making a blocking request for content that appears at the top of the acreen. (Or in the first screens worth of content) this has been shown to speed up the initial display of content." - Paul Kinlan