Skilling automatically adjusts to the small screens of mobile devices. If you're on a laptop or desktop, you can simulate a small screen by shrinking your browser window. See how, for example, the main menu changes, so it will fit on a small screen.
A warning, though. Good courses are active. For example, to encourage metacognition, give students two similar examples. Chunks of code, equations, whatever the course is about. Ask students to spot the differences and similarities between the two examples, and type their conclusions into the web page. Students will form an abstract schema, with "slots" for the differences in the examples.
(BTW, schema abstraction is a natural cognitive activity for humans. The Skilling lesson just prompts them to do it, and think it through by typing something in their own words.)
Students need a keyboard for this task. They can use a phone's virtual keyboard, but it's inconvenient, so they're less likely to do the task.
The base hardware students should use is a tablet, with a Bluetooth keyboard. You can get a good one on eBay (11" screen + keyboard) for about USD $70.
My dog Rosie uses an old Android tablet, with a Bluetooth keyboard. One of the keys is missing, but it works fine.
A laptop is better. A recent survey of online students found that 99% had laptops; even if fewer of your students have them, it will still be a big percentage. Windows, Mac, Linux... doesn't matter.