How to style a checkbox with the required attribute in HTML5

Question

Which of the following statements will not work as expected?

Accepted Answer

```css
[type=checkbox].cgu:required { border-color: red; }
```

Answer

```css
[type=text]:invalid { border-color: red; }
```

Answer

```css
[type=number]:out-of-range { border-color: red; }
```

Answer

```css
textarea:read-only { background-color: silver; color: gray; }
```

Question from the HTML5/CSS3 test