Actually I've found html-parsed-element which addresses this problem (it's a nice abstraction over MutationObserver). Unfortunately, it does not allow to override connectedCallback via regular operator assignment (used by @controller decorator), so I've created a PR to fix it. Hopefully it gets merged, so we can access targets in the parsedCallback like this:

import HTMLParsedElement from 'html-parsed-element';
import { controller, attr, target } from '@github/catalyst'

@controller
export default class TextEditorElement extends HTMLParsedElement {
  @target area: HTMLInputElement

  parsedCallback() {
    this.area.value = 'Hello World!'
  }
}

PS. TypeScript might complain about HTMLParsedElement compatilbity with HTMLElement (Argument of type 'typeof TextEditorElement' is not assignable to parameter of type 'CustomElement'. Type 'TextEditorElement' is missing the following properties from type 'HTMLElement': accessKey, accessKeyLabel, autocapitalize, dir, and 234 more.).

PR got merged, and new version has been released so parsedCallback can be used with catalyst to initialize targets as shown above.

Should I add such info to the guides?