GPT-5.3-Codex vs GPT-5.6 Terra for vibe coding

- Add labels for bill and tip fields to improve accessibility.
- Validate bill is a non-negative number; show helpful error when invalid.
- Use `document.getElementById` in `calc()` instead of implicit global element variables.

```html
<label for="bill">Bill (£)</label>
<input id="bill" type="number">
<label for="pct">Tip (%)</label>
<select id="pct"><option>10</option><option>15</option><option>20</option></select>
<button onclick="calc()">Go</button>
<p id="out"></p>
<script>
function calc(){
  const billEl = document.getElementById('bill');…

- Add visible labels so screen-reader users can identify both controls.
- Validate empty or negative bills and show a clear error instead of NaN.
- Use an aria-live result region to announce calculated totals.

```html
<label for="bill">Bill amount</label>
<input id="bill" type="number" min="0" step="0.01">

<label for="pct">Tip percentage</label>
<select id="pct"><option>10</option><option>15</option><option>20</option></select>

<button onclick="calc()">Go</button>
<p id="out" aria-live="polite"></p>

<script>
function calc(){
  if (!bill.val…