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4.10.18 Association of controls and forms

A form-associated element can have a relationship with a form element, which is called the element's form owner. If a form-associated element is not associated with a form element, its form owner is said to be null.

A form-associated element is, by default, associated with its nearest ancestor form element (as described below), but may have a form attribute specified to override this.

This feature allows authors to work around the lack of support for nested form elements.

If a form-associated element has a form attribute specified, then that attribute's value must be the ID of a form element in the element's owner Document.

The rules in this section are complicated by the fact that although conforming documents will never contain nested form elements, it is quite possible (e.g. using a script that performs DOM manipulation) to generate documents that have such nested elements. They are also complicated by rules in the HTML parser that, for historical reasons, can result in a form-associated element being associated with a form element that is not its ancestor.

When a form-associated element is created, its form owner must be initialized to null (no owner).

When a form-associated element is to be associated with a form, its form owner must be set to that form.

When a form-associated element or one of its ancestors is inserted into a Document, then the user agent must reset the form owner of that form-associated element. The HTML parser overrides this requirement when inserting form controls.

When an element is removed from a Document resulting in a form-associated element and its form owner (if any) no longer being in the same home subtree, then the user agent must reset the form owner of that form-associated element.

When a form-associated element's form attribute is set, changed, or removed, then the user agent must reset the form owner of that element.

When a form-associated element has a form attribute and the ID of any of the elements in the Document changes, then the user agent must reset the form owner of that form-associated element.

When a form-associated element has a form attribute and an element with an ID is inserted into or removed from the Document, then the user agent must reset the form owner of that form-associated element.

When the user agent is to reset the form owner of a form-associated element, it must run the following steps:

  1. If the element's form owner is not null, and the element's form content attribute is not present, and the element's form owner is its nearest form element ancestor after the change to the ancestor chain, then do nothing, and abort these steps.

  2. Let the element's form owner be null.

  3. If the element has a form content attribute and is itself in a Document, then run these substeps:

    1. If the first element in the Document to have an ID that is case-sensitively equal to the element's form content attribute's value is a form element, then associate the form-associated element with that form element.

    2. Abort the "reset the form owner" steps.

  4. Otherwise, if the form-associated element in question has an ancestor form element, then associate the form-associated element with the nearest such ancestor form element.

  5. Otherwise, the element is left unassociated.

In the following non-conforming snippet:

...
 <form id="a">
  <div id="b"></div>
 </form>
 <script>
  document.getElementById('b').innerHTML =
     '<table><tr><td><form id="c"><input id="d"></table>' +
     '<input id="e">';
 </script>
...

The form owner of "d" would be the inner nested form "c", while the form owner of "e" would be the outer form "a".

This happens as follows: First, the "e" node gets associated with "c" in the HTML parser. Then, the innerHTML algorithm moves the nodes from the temporary document to the "b" element. At this point, the nodes see their ancestor chain change, and thus all the "magic" associations done by the parser are reset to normal ancestor associations.

This example is a non-conforming document, though, as it is a violation of the content models to nest form elements.

element . form

Returns the element's form owner.

Returns null if there isn't one.

Form-associated elements have a form IDL attribute, which, on getting, must return the element's form owner, or null if there isn't one.