Inline CSS & JS
The majority of websites on the Internet today have separate HTML, CSS, and JS files.
Using in-line JS and CSS minimizes the back-and-forth queries necessary to display the website. However, it does make the HTML files longer. Also, parts of the code that exists within it will not get saved in the server cache. Ultimately, the call to go external or in-line remains up to the webmaster or development team of a particular website. With many other load time decreasing methods already in use, they may decide this is not worth it.
The best solution to optimize website speed performance is to use CSS inline only for the essential CSS styles in HTML documents.