Important information missing from the curriculum

I recently found this website and started the “Responsive Web Design” course. I completed HTML and CSS chapters and tried building a website on call with my friend. While doing the CSS part he pointed out that I was using internal css and using external css was a better practice. I researched about it and it seems like he was true and external css has alot of advantages. I clicked through the later chapters too and could not find it mentioned anywhere. I’m confused as to why it is not included in the curriculum if it is such a better practice.

The only difference between the two is if you put your HTML and CSS in the same file or separate files. This is demonstrated in the sample projects but is somewhat difficult to show in the challenges given how they are currently implemented.