My solutions are written in markdown. When you read them online here, GitHub Pages uses Jekyll to convert the markdown to nicely formatted HTML and displays that.
I used markdown because if you download the source files, you can read them, although the formatting is a bit odd. You can use an app that will display formatted markdown if you wish. You can also generate files in nicer formats (e.g., HTML) from the markdown source, which you can then save locally so you can read them offline.