It's a horrible mess so far. A quarter of the map is covered with a big explanatory panel, which can't b e closed. There are supposedly user ratings, but they can't be viewed. Many maps have no key or legend explaining what they show. A lot of clickable features do nothing.
It's tantalizing, but every map I went to was a disappointment. Much much better results just by going to Google Image search, and entering the word 'map' with what you want to see, and get dozens of hits to choose from.
Like this, for example, searching /map nebraska counties 1890/
https://www.google.com/search?q=map+...iw=909&bih=567