javascript - Why can't my code set attributes on elements?

Question

    <h1 class="MyClass">一</h1>
    <script>
        var hs =document.getElementsByTagName('h1');
        alert(hs[0].className);
        hs[0].className='XXXX';
    </script>

This is this code. Why can't I set a new value for this tag? setAttribute() doesn't work either.

Answer 1

Is there any interference from other h1 tags? I can use your code to test it and it’s fine

Answer 2

Are these the only codes? It should be fine. . .

Answer 3

Can be set.