What if we used this check instead of all three?

if (attrs.value==null) {
  attrs.value = attrs.defaultValue;
}

IMO, only undefined should be ignored.

if (attrs.defaultValue) {
	if (attrs.value === undefined) {
		attrs.value = attrs.defaultValue;
	}
	delete attrs.defaultValue;
}

actually, in check will be most strictly, but, preact-compat will merge defaultValue to value.

What is React's behavior here? They usually differentiate between null and undefined, but I haven't tested this specific behavior. We should just use whatever they use to avoid an inconsistency.

I also ran into problems with the preact-compat implementation of defaultValue.

In React, the defaultValue prop will not set the value prop on an element in general. In my observations, the value prop remains undefined. Changing the defaultValue prop does not cause any further changes/renders on the component.

The described behavior applies to the case of a select.