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chromium/chrome/test/data/extensions/api_test/mime_handler_view/test_page.html 

<!DOCTYPE html>
<title>Test page used for some MimeHandlerView tests</title>
<h1> Test Page</h1>

<script>
  function maybe_navigate(query) {
    const token = "?next=";
    if (!query.startsWith(token))
      return false;
    window.location.href = query.substr(token.length);
    return true;
  }

  function should_add_beforeunload(query) {
    if (query !== "?beforeunload")
      return false;
    window.addEventListener("beforeunload", (e) => {
        e.returnValue = "foo";
        return e;
    });
  }

  window.addEventListener("load", () => {
    let search = window.location.search;
    if (maybe_navigate(search))
      return;
    should_add_beforeunload(search);
  });
</script>