The web is built on HyperText Markup Language (HTML), proposed in 1990 as a way to solve an inherent problem with vast bodies of writing: referencing and creating links to other content.
The technology on HyperText has come a long way. Cascading Style Sheets (CSS) was proposed in 1994 bringing versatility to layout and visual aspects of HyperText documents; and in 1995 JavaScript came along to enable dynamic documents and complex behavior.
It didn't take two decades before these technologies would grow nearly limitless. Tools like D3 showcase the flexibility of the definition of what a "document" is, using HyperText blocks with styling and behavior to enable amazingly complex data visualization.
Yet most medium-to-long-form writing (like this one) is still mostly confined to print media presentation and structure, except for the occasional embedded interactive media, mostly supporting material. This seemingly staleness is not due to the lack of creativity. The person who coined the term "hypertext" had already proposed StretchText in 1967.
StretchText is an interesting and immediately appealing idea: text that can be expanded into more details if and only if the reader has interest on it. It keeps getting rediscovered, though never making it into wide adoption. Most people never heard of it and those who did have a hard time recalling it being used in more than 3 places.
Why is that? Such an intriguing and powerful idea with so little traction. There ought to be some reason to it. After writing some myself and reading it many times over, I think I can extrapolate from my own experience and the reasons why I'm probably not going to use it much more either.
When using stretchtext, in theory, one doesn't have to concern themselves with the task of reducing the overall scope to target a specific audience, as each level of interest and expertise should self guide to the appropriate level of detail.
In practice, that means writing for multiple possible audiences at the same time and planning how to weave those two (three, four...) texts into a single current of ideas. Instead of the usual segmentation mechanisms like foot notes, separate paragraphs, links to a separate document or a mere subordinate clause, there's now another set of choices to be made regarding how deep that information is hidden and how cohesive is the surrounding content with or without that segment.
Doesn't sound too hard, but stretching words into expanded forms within parent sentences requires writing similar to poetry. If stretching whole sentences to more sentences, there's still some effort in keeping harmony with the paragraph. This level of orchestration isn't expected from other forms of linked content and highly increases the rewriting and rewording loop.
Even if a reader is already optimally served by the current level of detail, they will expand deeper just to be sure. The reader cannot tell what is their ideal level of detail until they shoot past it, as there's usually no means of previewing the collapsed content. Worse yet, it's likely that most readers will expand the vast majority of collapsed content simply out of curiosity or FOMO anyway.
Readers that would otherwise have read the whole text can run out of patience for clicking and re-reading; and readers that would have benefited from the terser form will repeatedly experience the regret of unnecessary expansions before they learn to live with the anxiety of not knowing what's behind the curtain.
That's probably why many modern platforms offer preview popovers on mouse hover instead of stretchable embedded content: tooltip panes are cheap and ephemeral. The reader doesn't feel pressured to read or follow up content just because it popped up. We're used to dismissing popups. Popovers provide clearly demarcated context switching and the reader expects those to be related but self contained.
Given the big costs and difficulty of positive returns, the application of StretchText is probably destined to remain rare. Its alternatives like hyperlinks and conceptual successors like hovering tooltips will remain the preferred choices for tucking away additional content.
It also remains true that StretchText is enticing. It's more interactive, exercises curiosity, it's different. Because it's so difficult to get it right, when it clicks it's amazing. Used sparingly, intentionally and with a great deal of sense of art, it will keep getting rediscovered now and then as an exotic spice for hypertext. Arcane magic theorized before the web itself, a will of the ancients.