The `ui` package contains static files and templates used in the web UI. For easier distribution they are statically compiled into the Prometheus binary using the go-bindata tool (c.f. Makefile). During development it is more convenient to always use the files on disk to directly see changes without recompiling. Set the environment variable `DEBUG=1` and compile Prometheus for this to work. This is for development purposes only. After making changes to any file, run `make assets` before committing to update the generated inline version of the file.