Markdown

https://en.wikipedia.org/wiki/Markdown, made in 2004

It is the most popular a markup programming language, meaning it is a text-encoding system where certain words/sequences/characters will be displayed in a predefined way.

It is not the most popular from the aspect of best markup language, but moreso simply because it had traction in github README's and then became one of the standard documentation languages w/ the popular material for mkdocs static site generator template.

Nvim Rendering

Use plugin: https://dotfyle.com/plugins/iamcco/markdown-preview.nvim