I work on embedded firmware that has a web UI (as embedded firmwares have nowadays). I don't touch it much, but when I had to a major bit of development on it, I did as much work in JS, out of a big JSON generated by the firmware. One thing that allowed me to do was to capture the page put out by the firmware and then iterate on that, and work the changes back to the original source files. In fact, I accessed it as a local file (file:// URL). Otherwise it's: run the build to make the firmware image; flash; reboot; wait for firmware to bring up webUI server; navigate to the page, trigger some DOM event and wait". :)

Pretty sweet trick. I even think this should be a generalized strategy in computing and taught in studies.