CustomElementRegistry

Autonomous custom elements autonomous-custom-elements

Baseline: Widely available on the web, in every major browser since Jan 2020, and long enough to count on (since Jul 2022).

niso: Partial 80%. 2410 of 3010 subtests pass, by system: Linux 80%, macOS 80%. 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 · CustomElementRegistry, Window

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

The registry’s define, get, getName and whenDefined are the binding’s: they read the constructor and compare it by identity, then define it through the internal operations here with the callbacks they read from 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
Autonomous custom elements	Widely available	Partial 80% 2410 of 3010 subtests pass	3

Members

check()

operation not in JS

[NisoInternal] undefined check(DOMString name, DOMString? extends);

int32_t niso_custom_element_registry_check(niso_dom* dom, niso_handle self, const char* name, size_t name_len, const char* extends, size_t extends_len);

define()

operation not in JS

[NisoInternal] undefined define(DOMString name, DOMString? extends, CustomElementConstructor constructor, CustomElementUpgrade upgrade, CustomElementCallback? connectedCallback, CustomElementCallback? disconnectedCallback, CustomElementAttributeChangedCallback? attributeChangedCallback, sequence<DOMString> observedAttributes, sequence<DOMString> disabledFeatures, boolean formAssociated);

int32_t niso_custom_element_registry_define(niso_dom* dom, niso_handle self, const char* name, size_t name_len, const char* extends, size_t extends_len, niso_custom_element_constructor_fn constructor_fn, void* constructor_userdata, niso_drop_fn constructor_drop, niso_custom_element_upgrade_fn upgrade_fn, void* upgrade_userdata, niso_drop_fn upgrade_drop, niso_custom_element_callback_fn connected_callback_fn, void* connected_callback_userdata, niso_drop_fn connected_callback_drop, niso_custom_element_callback_fn disconnected_callback_fn, void* disconnected_callback_userdata, niso_drop_fn disconnected_callback_drop, niso_custom_element_attribute_changed_callback_fn attribute_changed_callback_fn, void* attribute_changed_callback_userdata, niso_drop_fn attribute_changed_callback_drop, const niso_str* observed_attributes, size_t observed_attributes_len, const niso_str* disabled_features, size_t disabled_features_len, bool form_associated);

upgrade()

operation

undefined upgrade(Node root);

int32_t niso_custom_element_registry_upgrade(niso_dom* dom, niso_handle self, niso_handle root);

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.

• CustomElementRegistry (Scoped custom element registries)
• builtin_element_support (Customized built-in elements)
• get (Autonomous custom elements)
• getName (Autonomous custom elements)
• initialize (Scoped custom element registries)
• whenDefined (Autonomous custom elements)