A <link rel=stylesheet href> appended via JS (createElement +
appendChild) was never fetched and fired neither load nor error, so any
Promise.all awaiting the link's onload (lazy CSS loaders, antd/bootstrap
fetchers) hung forever and the page stalled at its pre-load stage.
appendChild already drives this for <script src>; mirror it for links:
resolve the href against <base>/document URL, fetch it, and dispatch
load on success or error on failure. CSS is not applied (no layout
engine yet) but the load-event contract matches Chrome.
Read the stylesheet intent from both the rel attribute and the rel
property, since obscura does not yet reflect the rel IDL attribute back
to the content attribute. Closes#409.
SGavrl
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