HTMLFieldSetElement

<fieldset> and <legend> fieldset

Baseline: Widely available on the web, in every major browser since Jul 2015, and long enough to count on (since Jan 2018).

niso: Partial 63%. 209 of 328 subtests pass, by system: Linux 63%, macOS 63%. Engines: QuickJS, the engine niso's WPT run uses; Node and Porffor have no recorded run. WPT results recorded at f3b4fa8, 2026-11-10; web-features 3.45.0. WPT tests · niso's support · HTMLFieldSetElement, HTMLLegendElement

HTMLFieldSetElement is an interface in idl/html-elements.webidl, inheriting HTMLElement, exposed in Window. What it does is on MDN; this page says what niso has of it.

Status

The web features its members belong to, each with its Baseline status on the web and how much of the feature’s WPT suite passes in niso (see Web platform support).

Feature	On the web	In niso	Members
<fieldset> and <legend>	Widely available	Partial 63% 209 of 328 subtests pass	2

Members

constructor

constructor

[HTMLConstructor] constructor();

form

attribute

readonly attribute HTMLFormElement? form;

int32_t niso_html_field_set_element_get_form(niso_dom* dom, niso_handle self, niso_handle* out);

Not in niso yet

Members the web has, by the BCD keys web-features 3.45.0 lists, that idl/ doesn’t declare. The list is partial: web-features lists only the keys it maps to a feature.

• checkValidity (Constraint validation API)
• disabled (<fieldset> and <legend>)
• elements (<fieldset> and <legend>)
• name (<fieldset> and <legend>)
• reportValidity (Constraint validation API)
• setCustomValidity (Constraint validation API)
• type (<fieldset> and <legend>)
• validationMessage (Constraint validation API)
• validity (Constraint validation API)
• willValidate (Constraint validation API)