Lots of folks mentioning Markdown in the comments. +1 to that. Plain text FTW. I think a lot about my own data hoarding / archiving, and plain text is such a key part of that. Very future-proof.
Ever since WordPerfect I've preferred more deterministic, lightly-formatted documents with some way to see formatting characters directly. Markdown is brilliant, basically a DSL (domain-specific language) for HTML.
The key to plain text is tooling! A couple Markdown tools I haven't seen mentioned here yet (even though they've come up on HN before) are:
https://addons.mozilla.org/en-US/firefox/addon/markdown-view... - pretty-render Markdown right in the browser
https://casual-effects.com/markdeep/ - standalone web-friendly Markdown formatter with many features
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