It sounds like you are referring to the web-based version of Minecraft (often found on sites like minecraft.github.io or similar mirrors), which is typically an implementation of the Craftr or WebMCA projects. These are open-source attempts to bring the classic Minecraft experience to the browser using JavaScript and WebGL.
: A calculator that determines the cheapest XP cost for combining books and gear on an anvil to avoid the "Too Expensive!" error. MC Property Encyclopedia minecraft githubio full
It is important to approach these "github.io" sites with caution: It sounds like you are referring to the