ProleText Information is a document formatting standard supposedly used on ClariNet that stores text formatting information invisibly, so text readers not supporting the standard can still display the text legibly. The problem with HTML in email or newsgroup postings, for example, is that it looks like this when being read by a non-HTML client:

<h2>The Meaning of Life</h2><br><p>The cosmic balance in the universe is portrayed in <b>many</b> ways, such as in Pinkleton's essay <i>The Tired Sheep</i>...

ProleText moves its encoding information to the end of each line as a series of tabs and spaces, which often don't make a difference when viewed by a text-based email or news reader. A ProleText reader can use the extra spaces and tabs to show paragraphs, lists, and other formatted characters. Additionally, a formalized system of inline tags, such as *bold* and _italics or underline_, is supported in such a way that people reading encoded text with a non-ProleText reader can still understand that those represent bold and underlined text.

I don't know if this is still actively in use, but it's a neat form of backwards compatability that could still be useful in some contexts. See also the spec and an illustrated example.

(Found in the documentation for Acme::Bleach, ::DWIM and ::Morse, more examples of Damian Conway's wicked sense of humor.)