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chromium/third_party/blink/web_tests/external/wpt/contenteditable/plaintext-only.html 

<!DOCTYPE html>
<meta charset="utf-8">
<title>contenteditable="plaintext-only" should exist</title>
<script src="/resources/testharness.js"></script>
<script src="/resources/testharnessreport.js"></script>
<div id="txt" contenteditable="plaintext-only"></div>
<script>
test(function() {
    const txt = document.getElementById('txt');
    assert_true(txt.isContentEditable);
    assert_equals(txt.contentEditable, 'plaintext-only');
}, 'plaintext-only is an accepted attribute value for contenteditable');
test(function() {
    const txt = document.createElement('div');
    assert_false(txt.isContentEditable);
    txt.contentEditable = 'plaintext-only';
    assert_true(txt.isContentEditable);
    assert_equals(txt.contentEditable, 'plaintext-only');
}, 'plaintext-only can be assigned to contenteditable dynamically');
</script>