Ah sorry - disregard. I had a background process gobbling up GPU, and now I can use it in Firefox or Edge.
I do find it confusing, as I can put in numbers that get me well below a 3% withdrawal rate, but still have only 40% success rate. But I'm probably misunderstanding some of the mortgage sliders.
Yeah, it's confusing for sure. One day hopefully I'll have time to put a real UI on it. Right now it's assuming you buy a house with a mortgage (and property tax and yearly maintenance costs) at the same time as you retire. Put the house price to zero to eliminate all that. Also the expenses slider is before tax, so the tax rate slider influences how large your withdrawals actually are. The total withdrawal per year is displayed near the bottom.
I haven't tested the code extensively so I'm sure it's broken in some configurations. It's just a proof of concept demo for now.