Resource Pack HUD (otherwise known as RPHud) gives content creators the ability to mess with the in-game HUD without needing to write their own standalone mod. This is accomplished through the execution of RPHud scripting files, which are loaded from resource packs. Using the simple, quasi-functional language, it's easy for anybody with a rudimentary grasp of algebra to build a HUD like this:

Just about the same way you would use any other pack. Equip any RPHud-enabled pack, and the uppermost pack with RPHud support will be used to augment the HUD.
I'll probably build an index of RPHud packs if enough people are interested and enough packs are created. For now, though, you can find a few packs here:
Most of what you need can be found on the wiki. Also available is a Visual Studio Code extension that enables syntax highlighting and completion for RPHud script files.
No files available for download.