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chromium/third_party/blink/web_tests/editing/selection/mixed-editability-2.html 

<!doctype html>
<script src="../../resources/testharness.js"></script>
<script src="../../resources/testharnessreport.js"></script>
<script src="../assert_selection.js"></script>
<script>
selection_test(
  [
    '<div contenteditable>',
      'abc',
      '<b contenteditable="false" id="base">DEF</b>',
      'ghi<span id="extent">JKL</span>mno',
    '</div>',
  ],
  selection => selection.setBaseAndExtent(
        selection.document.getElementById('base'), 0,
        selection.document.getElementById('extent'), 0),
  [
    '<div contenteditable>',
      'abc',
      // Note: WebKit yields "^DEF|".
      '<b contenteditable="false" id="base">^DEF</b>',
      'ghi<span id="extent">|JKL</span>mno',
    '</div>',
  ],
  'Select non-editable to editable');
</script>