Q:

Please explain this CSS

I've noticed this code when I viewed page source:
<link rel="stylesheet" href="css:style.css" />

It doesn't make sense to me. "css" is not a valid URL to an external CSS file. The developer said something like this:
"It is reserved for the file > css/custom.css"
I still don't understand. Please, explain it to me if you know about such a code. 

A:

The css:style.css is probably a placeholder that will later get replaced with the actual URL to style.css by gulp, grunt or some other project automation tool.
It's a common practise with code reprocessors such as gulp-inject.

