In the code dropdown button in this page download the project as zip, extract it, then open Index.html and you're ready to select what you want to track.
Chose what pokemon generations you want to track and click Load selected, after it has loaded, you'll be shown the list of pokemon in the selected generations and you'll be able to set the amount you have of each pokemon, filter by name, filter obtained pokemon, show only a specific evolutionary line or download the current session as a file.
The last session is automatically saved, so you don't need to keep saving the last session manually if you don't intend to track different things each time.
While it's true that most pokemon games have a pokedex to keep track of what pokemon you have accuired, not all pokemon games have it (the mistery dungeon series for example), and not all of them keep track of the shinies you have accuired. This project is to help with those scenarios.
I wanted to keep the project simple so that it could just be downloded and run from any computer with a browser. Also, I already knew how to use jQuery ¯\_(ツ)_/¯
To those who created, contributed and maintain the Pokeapi(github) for saving me a lot of work on the pokemon data.