These files come from the [Equal Height Holy Grail Layout in Modern CSS](https://learnjsthehardway.com/blog/17-equal-height-holy-grail-layout-in-modern-css/) blog post.  The first layout described is in the file `basic.html`.  The two-tone layout is in `twotone.html`.  The quick simple mobile layout is in `mobile.html` and should probably the be actual one you use as the basis for your own study or design.