I feel like the article should've been called "plaintext-only websites" or something, because if you had asked me I would've also defined "text-only" as image/video-less websites
I struggle with the purity of meaning for text-only as well. Before this thread, I didn't understand the mime settings; I've been living a lie with a browser friendly landing page that uses:
<!DOCTYPE HTML><plaintext>
And then all the other pages of the site to be pure *.txt files. In the end, until there are standards to point to, I just accept minimalism as the scale. I have ads, layouts, boxes / frames, and all sorts of possibly annoying aspects to my textsites. It is a medium that's just as easily abused as any!
"No arbitrary code execution" is how I'd put it. "Ads" can be plain text, they just usually aren't on the internet. If a plain text site decided to include them once in a while, I'd celebrate the choice.
Fun to think of it but I think my website actually got removed from that list because it has a logo on top of each page. It is available as “text only” (although not text/plain but text/markdown) by substituting the .xhtml with .md in the URL unlike some other pages on the textonly.website list, though :)
To me, having only text as the output with no ads, videos, or images is “text-only”. It doesn’t matter how it’s presented as long as it’s just text.
But I also see your perspective. You want plain defaults with white background color, black foreground color, and no formatting.