Table of contents prototypes are done! Might add some non-critical polish in the upcoming days.

Fixed ToC w/ collapsible sections - https://di-toc-expandable.web.app/Hydrogen
Hiding ToC w/ dots - https://di-toc-floating.web.app/Vampire
Hiding ToC w/ header - https://di-toc-supplementary-2.web.app/Magnetic_field
Floating ToC in box - https://di-toc-supplementary.web.app/Sushi
Jump to ToC button - https://di-toc-back-to-top.web.app/Moss

FYI these prototypes only work in Chrome. You'll probably want to fix that if the plan is to share these more.

General: It's not clear to me what the plan for the table of the contents in the article is - will that be hidden for JS users? The existence of it in the prototypes helped cloud my experience as it made me aware of 2 table of contents.

Fixed ToC w/ collapsible sections - very intuitive to me, but was curious how this might work at lower resolutions for mobile and tablet (thinking in the grand scheme of things).
Hiding ToC w/ dots - not intuitive to me. Dots don't mean anything.
Hiding ToC w/ header - found I lost context. For instance, when viewing "The H-field" was not obvious right away how to get back to last section.
Floating ToC in box - found the experience disjointed given the table of contents in the article was in the left to start with and appeared in the bottom right.
Jump to ToC button - Almost missed this one, so not too intuitive. I like the simplicity of this solution, but to be honest I'd prefer a jump to top button than this. Doesn't help me jump back to the start of the article.