In WebKit, <details> was one of the first test-bed for (then non-exist) Web Components infrastructure. Before that, it directly manipulated the internal rendering tree, which is often error prone. Using higher-level primitive (Web Components) felt like a no-brainer but turned out to be a very tricky, took much longer to fresh it out. Because, after all, there was no such thing like Web Components!

Apparently this element is still implemented in Shadow-DOMy way [1]. So the effort was worth something after all, I guess.

[1] https://github.com/WebKit/WebKit/blob/main/Source/WebCore/ht...