Incredibly helpful! Notice they provide a zip of the code that you can run off-line as well.
Wow. I spent hours pouring over the HD44780 data sheet and couldn’t understand a thing. After 15 minutes of toying around with this thing, I had gained more knowledge about the interface than I had while reading the data sheet.
I guess it helps to have an free interactive interface where you can quickly and easily manipulate the bit data. Nice work!