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chromium/third_party/blink/web_tests/fast/forms/textarea/textarea-field-sizing-caret.html 

<!DOCTYPE html>
<script src="../../../resources/testharness.js"></script>
<script src="../../../resources/testharnessreport.js"></script>
<style>
textarea {
  border: none;
  padding: 0px;
  field-sizing: content;
}
</style>
<textarea></textarea>
<textarea style="writing-mode:vertical-rl"></textarea>
<script>
test(() => {
  assert_equals(document.querySelector('textarea').offsetWidth, 1);
}, 'The width of an empty textarea should be same as the caret width');

test(() => {
  assert_equals(document.querySelectorAll('textarea')[1].offsetHeight, 1);
}, 'The height of an empty textarea should be same as the caret height in a vertical wirting-mode');
</script>