What happens when a browser loads a website?
- It fetches the HTML page (index.html)
- Begin parsing the HTML
- The parser encounters a <script> tag referencing an external script file.
- The browser requests the script file. Meanwhile, the parser blocks and stops parsing the other HTML on your page.
- After some time the script is downloaded and subsequently executed.
- The parser continues parsing the rest of the HTML document.
(Step 4) It causes a bad user experience. Your website basically stops loading until you've downloaded all scripts. If there's one thing that users hate it's waiting for a website to load.