diff --git a/packages/scoped-elements/src/ScopedElementsMixin.js b/packages/scoped-elements/src/ScopedElementsMixin.js
index 55d5cd22..91588914 100644
--- a/packages/scoped-elements/src/ScopedElementsMixin.js
+++ b/packages/scoped-elements/src/ScopedElementsMixin.js
@@ -6,7 +6,9 @@ import { defineScopedElement, registerElement } from './registerElement.js';
 import { shadyTemplateFactory } from './shadyTemplateFactory.js';
 
 /**
+ * @typedef {import('./types').ScopedElementsMixin} ScopedElementsMixin
  * @typedef {import('./types').ScopedElementsMap} ScopedElementsMap
+ * @typedef {import("lit-element").LitElement} LitElement
  * @typedef {import('lit-html/lib/shady-render').ShadyRenderOptions} ShadyRenderOptions
  * @typedef {function(TemplateResult, Element|DocumentFragment|ShadowRoot, ShadyRenderOptions): void} RenderFunction
  */
@@ -111,64 +113,63 @@ const scopedElementsTemplateFactory = (
   return shadyTemplateFactory(scopeName)(newTemplate);
 };
 
-export const ScopedElementsMixin = dedupeMixin(
-  superclass =>
-    // eslint-disable-next-line no-shadow
-    class ScopedElementsMixin extends superclass {
-      /**
-       * Obtains the scoped elements definitions map
-       *
-       * @returns {ScopedElementsMap}
-       */
-      static get scopedElements() {
-        return {};
+/** @type {ScopedElementsMixin} */
+const ScopedElementsMixinImplementation = superclass =>
+  class ScopedElementsHost extends superclass {
+    /**
+     * Obtains the scoped elements definitions map
+     *
+     * @returns {ScopedElementsMap}
+     */
+    static get scopedElements() {
+      return {};
+    }
+
+    /** @override */
+    static render(template, container, options) {
+      if (!options || typeof options !== 'object' || !options.scopeName) {
+        throw new Error('The `scopeName` option is required.');
       }
+      const { scopeName } = options;
 
-      /** @override */
-      static render(template, container, options) {
-        if (!options || typeof options !== 'object' || !options.scopeName) {
-          throw new Error('The `scopeName` option is required.');
-        }
-        const { scopeName } = options;
+      const templateCache = getTemplateCache(this);
+      const tagsCache = getTagsCache(this);
+      const { scopedElements } = this;
 
-        const templateCache = getTemplateCache(this);
-        const tagsCache = getTagsCache(this);
-        const { scopedElements } = this;
+      return super.render(template, container, {
+        ...options,
+        templateFactory: scopedElementsTemplateFactory(
+          scopeName,
+          scopedElements,
+          templateCache,
+          tagsCache,
+        ),
+      });
+    }
 
-        // @ts-ignore
-        return super.render(template, container, {
-          ...options,
-          templateFactory: scopedElementsTemplateFactory(
-            scopeName,
-            scopedElements,
-            templateCache,
-            tagsCache,
-          ),
-        });
-      }
+    /**
+     * Defines a scoped element
+     *
+     * @param {string} tagName
+     * @param {typeof HTMLElement} klass
+     */
+    defineScopedElement(tagName, klass) {
+      return defineScopedElement(tagName, klass, getTagsCache(this.constructor));
+    }
 
-      /**
-       * Defines a scoped element
-       *
-       * @param {string} tagName
-       * @param {typeof HTMLElement} klass
-       */
-      defineScopedElement(tagName, klass) {
-        return defineScopedElement(tagName, klass, getTagsCache(this.constructor));
-      }
+    /**
+     * Returns a scoped tag name
+     *
+     * @param {string} tagName
+     * @returns {string|undefined}
+     */
+    static getScopedTagName(tagName) {
+      const klass = this.scopedElements[tagName];
 
-      /**
-       * Returns a scoped tag name
-       *
-       * @param {string} tagName
-       * @returns {string|undefined}
-       */
-      static getScopedTagName(tagName) {
-        const klass = this.scopedElements[tagName];
+      return klass
+        ? registerElement(tagName, klass, getTagsCache(this))
+        : getTagsCache(this).get(tagName);
+    }
+  };
 
-        return klass
-          ? registerElement(tagName, klass, getTagsCache(this))
-          : getTagsCache(this).get(tagName);
-      }
-    },
-);
+export const ScopedElementsMixin = dedupeMixin(ScopedElementsMixinImplementation);
diff --git a/packages/scoped-elements/src/types.d.ts b/packages/scoped-elements/src/types.d.ts
index 36967ba9..df457255 100644
--- a/packages/scoped-elements/src/types.d.ts
+++ b/packages/scoped-elements/src/types.d.ts
@@ -1,3 +1,27 @@
+import { Constructor } from "@open-wc/dedupe-mixin";
+import { LitElement } from "lit-element";
+
 export type ScopedElementsMap = {
     [key: string]: typeof HTMLElement;
 }
+
+export declare class ScopedElementsHost {
+  /**
+   * Obtains the scoped elements definitions map
+   */
+  static scopedElements: ScopedElementsMap;
+
+  /**
+   * Returns a scoped tag name
+   */
+  static getScopedTagName(tagName: string): string;
+
+  /**
+   * Defines a scoped element
+   */
+  defineScopedElement<T extends HTMLElement>(tagName: string, klass: Constructor<T>): void
+}
+
+declare function ScopedElementsMixinImplementation<T extends Constructor<LitElement>>(superclass: T): T & Constructor<ScopedElementsHost>
+
+export type ScopedElementsMixin = typeof ScopedElementsMixinImplementation;
diff --git a/packages/scoped-elements/test/ScopedElementsMixin.test.js b/packages/scoped-elements/test/ScopedElementsMixin.test.js
index 360141de..5f16d755 100644
--- a/packages/scoped-elements/test/ScopedElementsMixin.test.js
+++ b/packages/scoped-elements/test/ScopedElementsMixin.test.js
@@ -22,7 +22,6 @@ describe('ScopedElementsMixin', () => {
   it('has a default value for "static get scopedElements()" of {}', async () => {
     const tag = defineCE(class extends ScopedElementsMixin(LitElement) {});
     const el = await fixture(`<${tag}></${tag}>`);
-    // @ts-ignore
     expect(el.constructor.scopedElements).to.deep.equal({});
   });
 
@@ -257,7 +256,6 @@ describe('ScopedElementsMixin', () => {
     expect(el.shadowRoot.children[1]).to.not.be.an.instanceOf(FeatureB);
     expect(el.shadowRoot.children[2]).to.not.undefined;
 
-    // @ts-ignore
     el.defineScopedElement('feature-b', FeatureB);
 
     expect(el.shadowRoot.children[1]).to.be.an.instanceOf(FeatureB);
@@ -359,9 +357,7 @@ describe('ScopedElementsMixin', () => {
 
       const el = await fixture(`<${tag}></${tag}>`);
 
-      // @ts-ignore
       expect(el.constructor.getScopedTagName('feature-a')).to.match(tagRegExp);
-      // @ts-ignore
       expect(el.constructor.getScopedTagName('feature-b')).to.match(tagRegExp);
     });
 
@@ -387,7 +383,6 @@ describe('ScopedElementsMixin', () => {
 
       const el = await fixture(`<${tag}></${tag}>`);
 
-      // @ts-ignore
       expect(el.constructor.getScopedTagName('unregistered-feature')).to.match(tagRegExp);
     });
   });
diff --git a/packages/testing-helpers/src/litFixture.js b/packages/testing-helpers/src/litFixture.js
index c47ccc08..451e794b 100644
--- a/packages/testing-helpers/src/litFixture.js
+++ b/packages/testing-helpers/src/litFixture.js
@@ -7,7 +7,8 @@ import { getScopedElementsTemplate } from './scopedElementsWrapper.js';
 
 /**
  * @typedef {
-     import('lit-html').TemplateResult | import('lit-html').TemplateResult[]
+     import('lit-html').TemplateResult
+   | import('lit-html').TemplateResult[]
    | Node | Node[]
    | string | string[]
    | number | number[]
@@ -59,16 +60,20 @@ export function litFixtureSync(template, options = {}) {
  * @returns {Promise<T>}
  */
 export async function litFixture(template, options = {}) {
+  /** @type {T} */
+  // NB: in the case of scopedElements, this is ScopedElementsTestWrapper, not T,
+  // but that's only a small lie
   const el = litFixtureSync(template, options);
   await elementUpdated(el);
 
   if (options.scopedElements) {
-    const [node] = Array.from(el.shadowRoot.childNodes).filter(isUsefulNode);
-    await elementUpdated(/** @type {T} */ (node));
+    const [node] =
+      /** @type {T[]} */
+      (Array.from(el.shadowRoot.childNodes).filter(isUsefulNode));
+    await elementUpdated(node.firstElementChild);
 
-    return /** @type {T} */ (node);
+    return node;
   }
 
-  // @ts-ignore
   return el;
 }
diff --git a/packages/testing-helpers/src/scopedElementsWrapper.js b/packages/testing-helpers/src/scopedElementsWrapper.js
index 12b7a007..d70efb61 100644
--- a/packages/testing-helpers/src/scopedElementsWrapper.js
+++ b/packages/testing-helpers/src/scopedElementsWrapper.js
@@ -1,6 +1,5 @@
 import { ScopedElementsMixin } from '@open-wc/scoped-elements';
 import { html, LitElement, TemplateResult } from 'lit-element';
-import { nothing } from 'lit-html';
 import { isIterable } from './lib.js';
 
 /** @typedef {import('@open-wc/scoped-elements').ScopedElementsMap} ScopedElementsMap */
@@ -31,13 +30,13 @@ class ScopedElementsTestWrapper extends ScopedElementsMixin(LitElement) {
     /** @type {ScopedElementsMap} */
     this.scopedElements = {};
 
-    /** @type {TemplateResult|{}} */
-    this.template = nothing;
+    /** @type {import('./litFixture').LitHTMLRenderable} */
+    // eslint-disable-next-line babel/no-unused-expressions
+    this.template;
   }
 
-  async firstUpdated() {
-    // @ts-ignore
-    await super.firstUpdated();
+  firstUpdated(_changed) {
+    super.firstUpdated(_changed);
 
     Object.keys(this.scopedElements).forEach(key =>
       this.defineScopedElement(key, this.scopedElements[key]),
@@ -49,7 +48,6 @@ class ScopedElementsTestWrapper extends ScopedElementsMixin(LitElement) {
   }
 }
 
-// @ts-ignore
 customElements.define('scoped-elements-test-wrapper', ScopedElementsTestWrapper);
 
 /**