For some elements, the form attribute shouldn't be readonly

[KrishnaRPatel]

https://html.spec.whatwg.org/multipage/form-elements.html#the-output-element:dom-output-htmlfor

readonly attribute HTMLFormElement? form;

However, MDN details how form can be changed, specifically if the is located outside the

element:

The element to associate the output with (its form owner). The value of this attribute must be the id of a <form> in the same document. (If this attribute is not set, the <output> is associated with its ancestor element, if any.)
This attribute lets you associate <output> elements to <form>s anywhere in the document, not just inside a <form>. It can also override an ancestor <form> element.

Other elements that describe the form attribute as readonly while MDN describes how they can be set to a form's id include <input>, <button>, <textarea>, <fieldset>, and <object>.

If these attributes shouldn't be readonly, what would they be? Actually calling Element.form returns the linked form element so it does make sense for it to be treated as a readonly getter. On the other hand, it's misleading because by implementation, it can actually be set with a form id.

[domenic]

The form property on input/output/etc. elements in JavaScript isn't in direct correspondence to the form="" attribute in HTML. In particular, it gives the form owner, which is computed via a fairly complicated algorithm. Just as one example, if you nest an <input> element inside a <form> element, its form owner is the <form> element, even if its form="" attribute is omitted.

It's a bit unfortunate we have this mismatch, when normally the JS properties and the HTML attributes are in close correspondence. Ideally we would have called the JS property formOwner or something. Then maybe we could have a non-readonly form property that is in 1:1 correspondence with the form="" attribute. (But, maybe that'd be too confusing, if element.form does not return the actual <form> that owns the element.)

Overall, things are a bit unfortunate, but I don't think there's anything to change in the spec here. If you want to set the form="" attribute from JS, you can use element.setAttribute("form", "some-id"). As such, I'll close this issue, but we're happy to continue discussing in the closed thread.