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chromium/third_party/blink/web_tests/external/wpt/html/semantics/document-metadata/the-base-element/base-javascript.html 

<!-- Please update base-data.html together with this -->
<!DOCTYPE html>
<meta charset="utf-8">
<title>&lt;base> and javascript: URLs</title>
<script src=/resources/testharness.js></script>
<script src=/resources/testharnessreport.js></script>
<base href="javascript:/,test">
<base href="https://example.com/">
<div id=log></div>
<script>
test(() => {
  const link = document.createElement("a");
  link.href = "blah";
  assert_equals(link.href, new URL("blah", document.URL).href);
}, "First <base> has a javascript: URL so fallback is used");

test(() => {
  document.querySelector("base").remove();
  const link = document.createElement("a");
  link.href = "blah";
  assert_equals(link.href, new URL("blah", "https://example.com/").href);
}, "First <base> is removed so second is used");

test(() => {
  const base = document.createElement("base");
  base.href = "javascript:/,more-test";
  document.head.prepend(base);
  const link = document.createElement("a");
  link.href = "blah";
  assert_equals(link.href, new URL("blah", document.URL).href);
}, "Dynamically inserted first <base> has a javascript: URL so fallback is used");
</script>