A lot of native HTMLElement implementations are still missing in our fork of deno_dom.
The missing elements can be found in deno-dom/src/dom/types/tags.ts (all class names that are commented out).

To implement a new HTMLElement sub class (e.g. HTMLXElement), follow these steps (the order is important):

1. Make sure you have the latest version of uix with all submodules (uix-dom, deno_dom) recursively loaded
2. Create a new file called html-x-element.ts in deno-dom/src/dom/html-elements
3. Create a class that extends HTMLElement and add the required additional properties and methods following the HTML spec

   import { HTMLElement } from "../elements/html-element.ts";
   
   export class HTMLXElement extends HTMLElement {
      // custom implementation ...
   }

4. Export the newly created file from src/api.ts:

   export * from "./dom/html-elements/html-x-element.ts"

5. Add the correct type to deno-dom/src/dom/types/tags.ts (remove the element type that is commented out):

   import { HTMLXElement } from "../html-elements/html-x-element.ts";
   
   export interface HTMLElementTagNameMap {
     "x": HTMLXElement
   }

6. Add an element tag name mapping in uix-dom/dom/mod.ts

   elements.define("X", denoDom.HTMLXElement)

7. Add a global type definition in uix/src/app/dom-global.ts:

   const HTMLInputElement: typeof api.HTMLXElement
   type HTMLInputElement = api.HTMLXElement