Correctly ordering external stylesheets and external and inline scripts enables better parallelization of downloads and speeds up browser rendering time.
Because JavaScript code can alter the content and layout of a web page, the browser delays rendering any content that follows a script tag until that script has been downloaded, parsed and executed. However, more importantly for round-trip times, many browsers block the downloading of resources referenced in the document after scripts until those scripts are downloaded and executed. On the other hand, if other files are already in the process of being downloaded when a JS file is referenced, the JS file is downloaded in parallel with them. For example, let's say you have 3 stylesheets and 2 scripts and you specify them in the following order in the document