Markdown is a popular "easy-to-read, easy-to-write plain text format" that can be converted to the HTML web format, while still remaining easily readable as 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.