Comment by randomdata

Does he need to know how the Javascript files work, though? The HTML/CSS parts are still logically separated from the rest of the code. I am not sure someone who is capable of working on that type of content would struggle to recognize them. As long as the team is consistent, you can even reasonably predict exactly where such content would be found in the file.

If the designer was handed an HTML page with <style> tags embedded, I cannot imagine he would fall down unable to figure out where the CSS is, and this is not much different.