A proof-of-concept change that gets the most basic version of custom element overrides working.

src/compiler/compile/render_dom/wrappers/Element/index.ts

@@ -2,6 +2,7 @@ import Renderer from '../../Renderer';
 import Element from '../../../nodes/Element';
 import Wrapper from '../shared/Wrapper';
 import Block from '../../Block';
+import Text from '../../../nodes/Text';
 import { is_void, quote_prop_if_necessary, quote_name_if_necessary, sanitize } from '../../../../utils/names';
 import FragmentWrapper from '../Fragment';
 import { stringify, escape_html, escape } from '../../../utils/stringify';
@@ -374,6 +375,17 @@ export default class ElementWrapper extends Wrapper {
 
 	get_render_statement() {
 		const { name, namespace } = this.node;
+		let isList = this.node.attributes.filter(attr => attr.type === 'Attribute' && attr.name === 'is');
+		let is = null;
+		if (isList.length === 1) {
+			let isAttribute = isList[0];
+			
+			let chunk = isAttribute.chunks[0];
+			if (chunk.type === 'Text') {
+				let text = chunk as Text
+				is = text.data
+			}
+		}
 
 		if (namespace === 'http://www.w3.org/2000/svg') {
 			return `@svg_element("${name}")`;
@@ -383,6 +395,10 @@ export default class ElementWrapper extends Wrapper {
 			return `@_document.createElementNS("${namespace}", "${name}")`;
 		}
 
+		if (is) {
+			return `@element("${name}", "${is}")`;
+		}
+
 		return `@element("${name}")`;
 	}
 

src/runtime/internal/dom.ts

@@ -34,7 +34,10 @@ export function destroy_each(iterations, detaching) {
 	}
 }
 
-export function element<K extends keyof HTMLElementTagNameMap>(name: K) {
+export function element<K extends keyof HTMLElementTagNameMap>(name: K, is=null) {
+	if (is) {
+		return document.createElement<K>(name, { is })
+	}
 	return document.createElement<K>(name);
 }