Explorer Windows and Mac, Mozilla and Opera see <fieldset>s as form fields.
Mozilla and Opera allow change events on <fieldset>s, even
though they don't make sense.
Many browsers treat a <fieldset> as a form element, although
the spec
doesn't mention them in the list of control types.
document.forms.elements gives access to all form fields of a form. Explorer Windows,
Mozilla, and Opera also offer access to <fieldset>s, even though they're not
form fields.
When entering the page you saw an alert with all form fields your browser detected. As far
as I'm concerned the <fieldset> doesn't belong there.
I defined a change event on all form fields. Change the value of the inputs below,
and see which change events of which elements fire.
The event on the <fieldset> shouldn't fire, in fact, it shouldn't be there at all.
Nonetheless it fires in Mozilla and Opera.