Allow form validation on <output> element

The output element cannot have validation attributes:
https://html.spec.whatwg.org/multipage/form-elements.html#the-output-element

It could be useful to allow constraint validation. Use case:

<form oninput="result.value=parseInt(a.value)+parseInt(b.value)">
    <input type="number" id="a" name="a"> +
    <input type="number" id="b" name="b"> =
    <output name="result" for="a b"></output>
    <p>Total cannot be higher than 100</p>
</form>

There is no declarative way to set constraint validation on individual input elements in this case. With constraint validation on the output, authors could set max="100" and use native form validation and css :invalid selectors.

Start with the HTML Standard section linked in the issue, focusing on the output element and constraint validation. Define the required behavior for validation attributes, native form validation, and :invalid selectors; done means the standard clearly specifies and supports this use case.