This is the source code for the website at seriousbug.github.io/print-to-envelope.
It's a simple template for printing addresses on envelopes.
The code is open source and is licensed under MIT.
It's designed to work with Javascript disabled: it uses small amounts of Javascript for enhancements, but the core functionality works with Javascript disabled. The website also shows instructions on how to print without Javascript if you do have it disabled.