This is testing the ability for HTTP/2 to add Link Headers (in Nginx) that cause the browser to preload files. In this case, both the font file and image.css are not included in the HTML. Instead, nginx does Add-Headers to tell the browser to get them. When you click the "Load CSS" button, it adds the img.css link rel stylesheet to the head. The styles load instanly, because the browser has already downloaded them into the local cache. Basically it's like doing an HTML link rel preload, but by putting it in the Response headers, the browser can start fetching before it even starts parsing the head.