Luau
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Luau is a programming language forked from Lua 5.1 that has improved performance, a gradual type system, and added runtime features, and is used by for at least one IndieWeb site in development.
IndieWeb Examples
Rosalina Saige uses Luau for User:catgirlin.space/kitty.
Compatibility with Lua
The Luau website has some great documentation on compatibility with different versions of Lua along with some notes on reasons against why some features haven't been implemented. https://luau-lang.org/compatibility
Generally, anything written for Lua 5.1 will work with Luau, assuming it doesn't make use of libraries that have been removed for sandboxing. Stuff written for Luau that makes use of Luau-specific features like gradual typing will not work in Lua 5.1 without modifications. Projects like darklua can be used to help with this.
Brainstorming
Library needs
If anyone is up for it, there is a need for Luau libraries that support: