:disabled {
  /* ... */
}

<form action="#">
  <fieldset id="shipping">
    <legend>Shipping address</legend>
    <input type="text" placeholder="Name" />
    <input type="text" placeholder="Address" />
    <input type="text" placeholder="Zip Code" />
  </fieldset>
  <br />
  <fieldset id="billing">
    <legend>Billing address</legend>
    <label for="billing-checkbox">Same as shipping address:</label>
    <input type="checkbox" id="billing-checkbox" checked />
    <br />
    <input type="text" placeholder="Name" disabled />
    <input type="text" placeholder="Address" disabled />
    <input type="text" placeholder="Zip Code" disabled />
  </fieldset>
</form>

input[type="text"]:disabled {
  background: #ccc;
}

// Wait for the page to finish loading
document.addEventListener(
  "DOMContentLoaded",
  () => {
    // Attach `change` event listener to checkbox
    document.getElementById("billing-checkbox").onchange = toggleBilling;
  },
  false
);

function toggleBilling() {
  // Select the billing text fields
  const billingItems = document.querySelectorAll('#billing input[type="text"]');

  // Toggle the billing text fields
  billingItems.forEach((item) => {
    item.disabled = !item.disabled;
  });
}