One of the unexpected use of shadow DOMs for me was a document generated for image resource URLs [1], because the HTML standard apparently specifies the exact DOM structure of the generated document except for the `<head>` element [2].

[1] https://github.com/chromium/chromium/blob/f02ca734bf0/third_...

[2] https://html.spec.whatwg.org/multipage/document-lifecycle.ht...