Hidden text in plain text and Markdown files

A text file has no formatting to hide anything in, which makes it look safe. It is very often Markdown, though, and Markdown is very often rendered: a comment or a link title is in the file and on no page anybody reads. An AI model handed the file reads all of it.

We read a text file twice. Once as the text it is, and once as a browser would, so a span styled to be invisible is found even when it sits in the middle of a page of notes. Characters that draw nothing are covered on their own page, and apply here too.