I think endnotes are typically an awful idea, and the web can’t do footnotes. What you want is generally side notes. I think what I do on my website is a fairly good compromise for JavaScript-free operation, with a side column for notes on large enough screens, and the notes inlined on small screens. That wouldn’t be suitable for very long notes; such are often more suitable as appendixes, a variant on endnotes.
As for <details>, you run into the problem that it’s a block-level element; it’s dubious using it as an inline-level element, though it’ll probably work well enough (I say probably due to uncertainty about screen readers) despite being nominally invalid, given that it’s not an element that will automatically close a paragraph tag like <div> does.
I think links at the bottom is generally foolish, taking more effort for both reader and writer, and never do it that way, interspersing them in the text, usually surrounded by angle brackets as has historically been the way of delimiting URLs in plain text.
As for <details>, you run into the problem that it’s a block-level element; it’s dubious using it as an inline-level element, though it’ll probably work well enough (I say probably due to uncertainty about screen readers) despite being nominally invalid, given that it’s not an element that will automatically close a paragraph tag like <div> does.
I think links at the bottom is generally foolish, taking more effort for both reader and writer, and never do it that way, interspersing them in the text, usually surrounded by angle brackets as has historically been the way of delimiting URLs in plain text.