These stairs are just a trial set, to see if we like the way they work. To make sure they aren't too steep, don't take up too much room in the interior, etc. The treads are scrap 3/4 osb subfloor. A bit spongy, but ok for their purpose. I certainly wouldn't jump on them. The stairs are 32" wide.
I envision making the final ones in a similar fashion, only with metal stringers. And more solid treads.