Markdown is a popular "easy-to-read, easy-to-write plain text format" that can be converted to a visually formatted HTML web document, while still remaining easily readable as a plain text.
The tiny bullets in this table mark compatible extensions to classic Markdown.
For more Markdown options, see for instance Markdown Guide.
TeX is a popular plain text format for scientific formulae. While most blogs, social media, and other online publishing platforms won't allow for insertion of human-readable formulae, pencraft.space will make your formulae shine.