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<p>The :indeterminate pseudo-class applies to UI elements whose value is in an indeterminate state. In this page <mark style="background:coral"><u>coral and underline(border) is indeterminate style</u></mark>.</p>

<h2>Checkbox state demo</h2>
  <fieldset id="checkbox-field">
    <legend>true / false / indeterminate state</legend>
    <input type="checkbox" id="checkbox-true" checked>
    <label for="checkbox-true">TRUE</label>
    <input type="checkbox" id="checkbox-false">
    <label for="checkbox-false">FALSE</label>
    <input type="checkbox" id="checkbox-indeterminate">
    <label for="checkbox-indeterminate">INDETERMINATE</label>
    ⬅️ <code>document.getElementById('...').indeterminate = true;</code>
  <button id="checkbox-reset" type="reset">Reset checkbox</button>

<h2>Radio group state demo</h2>
  <fieldset id="radio-determinate-field">
    <legend>determinate state</legend>
    <input type="radio" id="radio-determinate-true" name="radio" checked>
    <label for="radio-determinate-true">TRUE</label>
    <input type="radio" id="radio-determinate-false" name="radio">
    <label for="radio-determinate-false">FALSE</label>
  <fieldset id="radio-indeterminate-field">
    <legend>indeterminate state</legend>
    <input type="radio" id="radio-indeterminate-true" name="radio-indeterminate">
    <label for="radio-indeterminate-true">TRUE</label>
    <input type="radio" id="radio-indeterminate-false" name="radio-indeterminate">
    <label for="radio-indeterminate-false">FALSE</label>
  <button id="radio-reset" type="reset">Reset radio</button>

<h2>Progress state demo</h2>
<p>determinate state: <progress value=".5">50%</progress> ⬅️ <code>&lt;progress value=".5"&gt;50%&lt;/progress&gt;</code></p>
<p>indeterminate state: <progress>Not known</progress> ⬅️ <code>&lt;progress&gt;Not known&lt;/progress&gt;</code></p>



                input:indeterminate + label { /* input + label indeterminate style */
  text-decoration: underline;
  background: coral;

progress { -webkit-appearance: none; appearance: none; }
progress::-webkit-progress-bar { background: silver; }
progress::-webkit-progress-value { background: blue; }

progress:indeterminate::-webkit-progress-bar { /* progress indeterminate style */
  background: coral;
  border-bottom: 1px solid;



                document.getElementById('checkbox-indeterminate').indeterminate = true;

document.getElementById('checkbox-reset').addEventListener('click', () => {
  document.getElementById('checkbox-indeterminate').indeterminate = true;