Modern browsers have improved their ability to load resources in parallel, but scripts still follow the order they appear on the page. This advice is more relevant for older browsers. A recommended practice is to move script tags near the end of the body tag to ensure the page renders before executing JavaScript. Google's Best Practices recommend this approach: https://developers.google.com/apps-script/guides/html/best-practices#load_javascript_last
Additionally, leveraging a Content Delivery Network (CDN) can significantly boost performance. By loading scripts from a CDN, users may already have cached those files from visiting other sites, eliminating the need to download them again from your server. Furthermore, downloading multiple scripts from different sources simultaneously can be delayed when loading from your server, making CDN loading a beneficial option.
In relation to the outdated video:
The video suggests that CSS files will only start downloading after script tags if they precede them. Conversely, if script tags are loading, CSS files won't begin loading until all scripts are loaded.
To elaborate further, imagine that once a script starts loading, nothing else can start loading. However, if a CSS file begins loading, the next script can commence loading concurrently with the CSS file. In this scenario, both resources load simultaneously. But if a script initiates loading, the CSS file cannot start loading until the script is fully loaded.