https://4thmix.plus/p/1-stepcharts

This is still in heavy beta so certain things may not work or look correct!

doubles mode really requires desktop or a tablet to fit
older phones may not be able to view the entire viewport (the UI is a bit big)
touch input does not work for scrubbing through the chart (though it does on desktop)

the UI is (mostly) designed with desktop view in mind but it works on mobile with some concessions.

almost none of this code is mine, smeditor is entirely FOSS and you can use here independently of the site: https://tillvit.github.io/smeditor/
there are many more options available, I disable some features for the sake of just embedding playback.
tillvit is an absolute legend for creating this.

I also found some ages-old PHP code for parsing SM files which works (enough) to do some basic parsing on the backend as well: https://github.com/jasonwatt/SMParser so shoutouts to them for saving me a bunch of time.

all charts were grabbed from ~~DDRUK~~ zenius-i-vanisher, so, thanks to them for keeping an updated database of simfiles I can use.