What you are looking at is a custom gunpla backlog inventory management and random kit picker application written in HTML.  I'm still working out some of the kinks.  Want to give it a try?  

TO START:  Click on Manage Inventory in the top-right corner.  When in the Inventory Manager, click RESET TO DEFAULT, and then click OK.  This is my backlog, as of a few months ago.  To start your own, click on the CLEAR ALL button.  Then once you've added your kits, click SET AS DEFAULT.  It works using your browser cookies, so if you clear them your backlog is gone.  You can also export your backlog into a .csv (excel spreadsheet file), and then reload it later by importing it in.  If you have any questions, contact me on discord!