In T31118#9042921, @cscott wrote:

There's a long discussion in the parent task, T25932, summarized by "wikitext is not a superset of HTML". If we want to allow markup like:

<details open>
<summary>(short description)</summary>
;Name
:Lorem Ipsum
</details>

then the best way forward (in terms of future maintainability) is not to "just" allow arbitrary use of <details> and <summary> tags in wikitext, but instead to start by adding a new parser function, say {{#details|short description|content}} which will emit the structure that you want. (The 'content' block is a good use of heredoc arguments, T114432.) That is "more semantic" in tat the wikitext author is writing a semantic description of the details box (or whatever), and it allows us the ability to change the exact markup emitted for that parser function in the future. We're not committed to emitting exactly those particular HTML tags in that particular tree structure for all time.

Why can't the entire text be rendered at load time? As long as the browser doesn't run out of memory and crash, it should work fine. I would personally find infinite scroll annoying if for example I were reading Wikisource on an unstable connection.