diff --git a/.gitignore b/.gitignore
index 9d37538..732d377 100644
--- a/.gitignore
+++ b/.gitignore
@@ -4,3 +4,4 @@ _site
 lib/
 .jekyll-cache
 .lighthouseci
+coverage
diff --git a/package.json b/package.json
index c76177a..b8342ca 100644
--- a/package.json
+++ b/package.json
@@ -23,10 +23,11 @@
     "lib"
   ],
   "scripts": {
-    "build": "tsc --build",
+    "build": "tsc --build tsconfig.build.json",
     "build:docs": "cd docs && JEKYLL_ENV=production bundle exec jekyll build",
-    "clean": "tsc --build --clean",
+    "clean": "tsc --build --clean tsconfig.build.json",
     "lint": "eslint . --ignore-path .gitignore",
+    "postlint": "tsc",
     "prepack": "npm run build",
     "presize": "npm run build",
     "size": "size-limit",
diff --git a/src/mark.ts b/src/mark.ts
index 045476b..b1caf53 100644
--- a/src/mark.ts
+++ b/src/mark.ts
@@ -1,9 +1,11 @@
 type PropertyType = 'field' | 'getter' | 'setter' | 'method'
-type PropertyDecorator = (proto: object, key: PropertyKey) => void
+interface PropertyDecorator {
+  (proto: object, key: PropertyKey, descriptor?: PropertyDescriptor): void
+  readonly static: unique symbol
+}
 type GetMarks = (instance: object) => Set<PropertyKey>
 export function createMark(validate: (key: PropertyKey, type: PropertyType) => void): [PropertyDecorator, GetMarks] {
   const marks = new WeakMap<object, Set<PropertyKey>>()
-  const sym = Symbol()
   function get(proto: object): Set<PropertyKey> {
     if (!marks.has(proto)) {
       const parent = Object.getPrototypeOf(proto)
@@ -22,13 +24,15 @@ export function createMark(validate: (key: PropertyKey, type: PropertyType) => v
     validate(key, type)
     get(proto).add(key)
   }
-  marker.static = sym
+  marker.static = Symbol()
 
   return [
-    marker,
+    marker as PropertyDecorator,
     (instance: object): Set<PropertyKey> => {
       const proto = Object.getPrototypeOf(instance)
-      for (const key of proto.constructor[sym] || []) marker(proto, key, Object.getOwnPropertyDescriptor(proto, key))
+      for (const key of proto.constructor[marker.static] || []) {
+        marker(proto, key, Object.getOwnPropertyDescriptor(proto, key))
+      }
       return new Set(get(proto))
     }
   ]
diff --git a/test/ability.ts b/test/ability.ts
index 12c2fb9..222ec12 100644
--- a/test/ability.ts
+++ b/test/ability.ts
@@ -1,9 +1,10 @@
 import {expect, fixture, html} from '@open-wc/testing'
 import {restore, fake} from 'sinon'
+import type {CustomElement} from '../src/custom-element.js'
 import {createAbility, attachShadowCallback, attachInternalsCallback} from '../src/ability.js'
 
 describe('ability', () => {
-  let calls = []
+  let calls: string[] = []
   const fakeable = createAbility(
     Class =>
       class extends Class {
@@ -22,9 +23,9 @@ describe('ability', () => {
           calls.push('fakeable adoptedCallback')
           super.adoptedCallback?.()
         }
-        attributeChangedCallback(...args) {
+        attributeChangedCallback(name: string, oldValue: string | null, newValue: string | null) {
           calls.push('fakeable attributeChangedCallback')
-          super.attributeChangedCallback?.(...args)
+          super.attributeChangedCallback?.(name, oldValue, newValue)
         }
       }
   )
@@ -46,9 +47,9 @@ describe('ability', () => {
           calls.push('otherfakeable adoptedCallback')
           super.adoptedCallback?.()
         }
-        attributeChangedCallback(...args) {
+        attributeChangedCallback(name: string, oldValue: string | null, newValue: string | null) {
           calls.push('otherfakeable attributeChangedCallback')
-          super.attributeChangedCallback?.(...args)
+          super.attributeChangedCallback?.(name, oldValue, newValue)
         }
       }
   )
@@ -85,9 +86,9 @@ describe('ability', () => {
   it('can be called multiple times, but only applies once', async () => {
     const MultipleFakeable = fakeable(fakeable(fakeable(fakeable(fakeable(Element)))))
     customElements.define('multiple-fakeable', MultipleFakeable)
-    const instance = await fixture(html`<multiple-fakeable />`)
+    const instance: CustomElement = await fixture(html`<multiple-fakeable />`)
     expect(calls).to.eql(['fakeable connectedCallback'])
-    instance.connectedCallback()
+    instance.connectedCallback!()
     expect(calls).to.eql(['fakeable connectedCallback', 'fakeable connectedCallback'])
   })
 
@@ -95,22 +96,22 @@ describe('ability', () => {
     const CoreTest = otherfakeable(fakeable(Element))
     customElements.define('core-test', CoreTest)
 
-    let instance
+    let instance: CustomElement & typeof CoreTest
     beforeEach(async () => {
       instance = await fixture(html`<core-test />`)
     })
 
     it('applies keys from delegate onto subclass upon instantiation', () => {
       expect(instance).to.have.property('foo')
-      expect(instance.foo()).to.equal('foo!')
+      expect((instance as unknown as Record<string, () => void>).foo()).to.equal('foo!')
       expect(instance).to.have.property('bar')
-      expect(instance.bar()).to.equal('bar!')
+      expect((instance as unknown as Record<string, () => void>).bar()).to.equal('bar!')
     })
 
     for (const method of ['connectedCallback', 'disconnectedCallback', 'adoptedCallback', 'attributeChangedCallback']) {
       it(`delegates to other ${method}s before class ${method}`, () => {
         calls = []
-        instance[method]()
+        ;(instance as unknown as Record<string, () => void>)[method]()
         expect(calls).to.eql([`otherfakeable ${method}`, `fakeable ${method}`])
       })
     }
@@ -118,8 +119,8 @@ describe('ability', () => {
 
   describe('ability extension behaviour', () => {
     describe('attachShadowCallback', () => {
-      let attachShadowFake
-      let shadow
+      let attachShadowFake: (shadow: ShadowRoot) => void
+      let shadow: ShadowRoot | null
       beforeEach(() => {
         shadow = null
         attachShadowFake = fake()
@@ -128,8 +129,8 @@ describe('ability', () => {
       const declarable = createAbility(
         Class =>
           class extends Class {
-            [attachShadowCallback](...args) {
-              super[attachShadowCallback](...args)
+            [attachShadowCallback](...args: [ShadowRoot]) {
+              super[attachShadowCallback]!(...args)
               return attachShadowFake.apply(this, args)
             }
           }
@@ -211,8 +212,8 @@ describe('ability', () => {
     })
 
     describe('attachInternalsCallback', () => {
-      let attachInternalsFake
-      let internals
+      let attachInternalsFake: (internals: ElementInternals) => void
+      let internals: ElementInternals | null
       beforeEach(() => {
         internals = null
         attachInternalsFake = fake()
@@ -221,8 +222,8 @@ describe('ability', () => {
       const internable = createAbility(
         Class =>
           class extends Class {
-            [attachInternalsCallback](...args) {
-              super[attachInternalsCallback](...args)
+            [attachInternalsCallback](...args: [ElementInternals]) {
+              super[attachInternalsCallback]!(...args)
               return attachInternalsFake.apply(this, args)
             }
           }
@@ -261,7 +262,7 @@ describe('ability', () => {
       })
 
       it('errors if userland calls attachInternals more than once', async () => {
-        const instance = await fixture(html`<manual-internals-ability></manual-internals-ability>`)
+        const instance = await fixture<CustomElement>(html`<manual-internals-ability></manual-internals-ability>`)
         internals = instance.attachInternals()
         expect(internals).to.exist.and.be.instanceof(ElementInternals)
         expect(attachInternalsFake).to.be.calledOnce.calledOn(instance).and.calledWithExactly(internals)
diff --git a/test/attr.ts b/test/attr.ts
index 1b262c2..211bbda 100644
--- a/test/attr.ts
+++ b/test/attr.ts
@@ -3,79 +3,79 @@ import {controller} from '../src/controller.js'
 import {attr} from '../src/attr.js'
 
 describe('Attr', () => {
-  @controller
-  // eslint-disable-next-line @typescript-eslint/no-unused-vars
-  class InitializeAttrTest extends HTMLElement {
-    @attr fooBar = 'hello'
-    fooBaz = 1
+  {
+    @controller
+    class InitializeAttrTest extends HTMLElement {
+      @attr fooBar = 'hello'
+      fooBaz = 1
 
-    getCount = 0
-    setCount = 0
-    #bing = 'world'
-    get bingBaz() {
-      this.getCount += 1
-      return this.#bing
-    }
-    @attr set bingBaz(value: string) {
-      this.setCount += 1
-      this.#bing = value
+      getCount = 0
+      setCount = 0
+      #bing = 'world'
+      get bingBaz() {
+        this.getCount += 1
+        return this.#bing
+      }
+      @attr set bingBaz(value: string) {
+        this.setCount += 1
+        this.#bing = value
+      }
     }
+
+    let instance: InitializeAttrTest
+    beforeEach(async () => {
+      instance = await fixture(html`<initialize-attr-test />`)
+    })
+
+    it('does not error during creation', () => {
+      document.createElement('initialize-attr-test')
+    })
+
+    it('does not alter field values from their initial value', () => {
+      expect(instance).to.have.property('fooBar', 'hello')
+      expect(instance).to.have.property('fooBaz', 1)
+      expect(instance).to.have.property('bingBaz', 'world')
+    })
+
+    it('reflects the initial value as an attribute, if not present', () => {
+      expect(instance).to.have.attribute('data-foo-bar', 'hello')
+      expect(instance).to.not.have.attribute('data-foo-baz')
+      expect(instance).to.have.attribute('data-bing-baz', 'world')
+    })
+
+    it('prioritises the value in the attribute over the property', async () => {
+      instance = await fixture(html`<initialize-attr-test data-foo-bar="goodbye" data-bing-baz="universe" />`)
+      expect(instance).to.have.property('fooBar', 'goodbye')
+      expect(instance).to.have.attribute('data-foo-bar', 'goodbye')
+      expect(instance).to.have.property('bingBaz', 'universe')
+      expect(instance).to.have.attribute('data-bing-baz', 'universe')
+    })
+
+    it('changes the property when the attribute changes', async () => {
+      instance.setAttribute('data-foo-bar', 'goodbye')
+      await Promise.resolve()
+      expect(instance).to.have.property('fooBar', 'goodbye')
+      instance.setAttribute('data-bing-baz', 'universe')
+      await Promise.resolve()
+      expect(instance).to.have.property('bingBaz', 'universe')
+    })
+
+    it('changes the attribute when the property changes', () => {
+      instance.fooBar = 'goodbye'
+      expect(instance).to.have.attribute('data-foo-bar', 'goodbye')
+      instance.bingBaz = 'universe'
+      expect(instance).to.have.attribute('data-bing-baz', 'universe')
+    })
   }
 
-  let instance
-  beforeEach(async () => {
-    instance = await fixture(html`<initialize-attr-test />`)
-  })
-
-  it('does not error during creation', () => {
-    document.createElement('initialize-attr-test')
-  })
-
-  it('does not alter field values from their initial value', () => {
-    expect(instance).to.have.property('fooBar', 'hello')
-    expect(instance).to.have.property('fooBaz', 1)
-    expect(instance).to.have.property('bingBaz', 'world')
-  })
-
-  it('reflects the initial value as an attribute, if not present', () => {
-    expect(instance).to.have.attribute('data-foo-bar', 'hello')
-    expect(instance).to.not.have.attribute('data-foo-baz')
-    expect(instance).to.have.attribute('data-bing-baz', 'world')
-  })
-
-  it('prioritises the value in the attribute over the property', async () => {
-    instance = await fixture(html`<initialize-attr-test data-foo-bar="goodbye" data-bing-baz="universe" />`)
-    expect(instance).to.have.property('fooBar', 'goodbye')
-    expect(instance).to.have.attribute('data-foo-bar', 'goodbye')
-    expect(instance).to.have.property('bingBaz', 'universe')
-    expect(instance).to.have.attribute('data-bing-baz', 'universe')
-  })
-
-  it('changes the property when the attribute changes', async () => {
-    instance.setAttribute('data-foo-bar', 'goodbye')
-    await Promise.resolve()
-    expect(instance).to.have.property('fooBar', 'goodbye')
-    instance.setAttribute('data-bing-baz', 'universe')
-    await Promise.resolve()
-    expect(instance).to.have.property('bingBaz', 'universe')
-  })
-
-  it('changes the attribute when the property changes', () => {
-    instance.fooBar = 'goodbye'
-    expect(instance).to.have.attribute('data-foo-bar', 'goodbye')
-    instance.bingBaz = 'universe'
-    expect(instance).to.have.attribute('data-bing-baz', 'universe')
-  })
-
   describe('types', () => {
     it('infers boolean types from property and uses has/toggleAttribute', async () => {
       @controller
-      // eslint-disable-next-line @typescript-eslint/no-unused-vars
       class BooleanAttrTest extends HTMLElement {
         @attr fooBar = false
       }
 
-      instance = await fixture(html`<boolean-attr-test />`)
+      const instance = await fixture<BooleanAttrTest>(html`<boolean-attr-test />`)
 
       expect(instance).to.have.property('fooBar', false)
       expect(instance).to.not.have.attribute('data-foo-bar')
@@ -104,7 +104,6 @@ describe('Attr', () => {
 
     it('avoids infinite loops', async () => {
       @controller
-      // eslint-disable-next-line @typescript-eslint/no-unused-vars
       class LoopAttrTest extends HTMLElement {
         count = 0
         @attr
@@ -115,7 +114,8 @@ describe('Attr', () => {
           this.count += 1
         }
       }
-      instance = await fixture(html`<loop-attr-test />`)
+
+      const instance = await fixture<LoopAttrTest>(html`<loop-attr-test />`)
 
       expect(instance).to.have.property('fooBar')
       instance.fooBar = 1
@@ -127,13 +127,13 @@ describe('Attr', () => {
 
   describe('naming', () => {
     @controller
-    // eslint-disable-next-line @typescript-eslint/no-unused-vars
     class NamingAttrTest extends HTMLElement {
       @attr fooBarBazBing = 'a'
       @attr URLBar = 'b'
       @attr ClipX = 'c'
     }
 
+    let instance: NamingAttrTest
     beforeEach(async () => {
       instance = await fixture(html`<naming-attr-test />`)
     })
diff --git a/test/auto-shadow-root.ts b/test/auto-shadow-root.ts
index 776f11c..221c3da 100644
--- a/test/auto-shadow-root.ts
+++ b/test/auto-shadow-root.ts
@@ -3,9 +3,12 @@ import {replace, fake} from 'sinon'
 import {autoShadowRoot} from '../src/auto-shadow-root.js'
 
 describe('autoShadowRoot', () => {
-  window.customElements.define('shadowroot-test-element', class extends HTMLElement {})
+  class ShadowRootTestElement extends HTMLElement {
+    declare shadowRoot: ShadowRoot
+  }
+  window.customElements.define('shadowroot-test-element', ShadowRootTestElement)
 
-  let instance
+  let instance: ShadowRootTestElement
   beforeEach(async () => {
     instance = await fixture(html`<shadowroot-test-element />`)
   })
@@ -58,7 +61,7 @@ describe('autoShadowRoot', () => {
     instance = await fixture(html`<shadowroot-test-element>
       <template data-shadowroot="closed">Hello World</template>
     </shadowroot-test-element>`)
-    let shadowRoot = null
+    let shadowRoot: ShadowRoot | null = null
     replace(
       instance,
       'attachShadow',
diff --git a/test/bind.ts b/test/bind.ts
index 217515b..b329061 100644
--- a/test/bind.ts
+++ b/test/bind.ts
@@ -3,18 +3,16 @@ import {replace, fake} from 'sinon'
 import {bind, listenForBind} from '../src/bind.js'
 
 describe('bind', () => {
-  window.customElements.define(
-    'bind-test-element',
-    class extends HTMLElement {
-      foo = fake()
-      bar = fake()
-      handleEvent = fake()
-    }
-  )
+  class BindTestElement extends HTMLElement {
+    foo = fake()
+    bar = fake()
+    handleEvent = fake()
+  }
+  window.customElements.define('bind-test-element', BindTestElement)
 
-  let instance
+  let instance: BindTestElement
   beforeEach(async () => {
-    instance = await fixture(html`<bind-test-element />`)
+    instance = await fixture<BindTestElement>(html`<bind-test-element />`)
   })
 
   it('binds events on elements based on their data-action attribute', () => {
@@ -47,7 +45,7 @@ describe('bind', () => {
 
   it('does not bind elements whose closest selector is not this controller', () => {
     const el = document.createElement('div')
-    el.getAttribute('data-action', 'click:bind-test-element#foo')
+    el.setAttribute('data-action', 'click:bind-test-element#foo')
     const container = document.createElement('div')
     container.append(instance, el)
     bind(instance)
@@ -157,7 +155,7 @@ describe('bind', () => {
     const el2 = document.createElement('div')
     el1.setAttribute('data-action', 'click:bind-test-element#foo')
     el2.setAttribute('data-action', 'submit:bind-test-element#foo')
-    instance.shadowRoot.append(el1, el2)
+    instance.shadowRoot!.append(el1, el2)
     bind(instance)
     expect(instance.foo).to.have.callCount(0)
     el1.click()
@@ -173,8 +171,8 @@ describe('bind', () => {
     el1.setAttribute('data-action', 'click:bind-test-element#foo')
     el2.setAttribute('data-action', 'submit:bind-test-element#foo')
     bind(instance)
-    instance.shadowRoot.append(el1)
-    instance.shadowRoot.append(el2)
+    instance.shadowRoot!.append(el1)
+    instance.shadowRoot!.append(el2)
     // We need to wait for one microtask after injecting the HTML into to
     // controller so that the actions have been bound to the controller.
     await Promise.resolve()
@@ -267,7 +265,7 @@ describe('bind', () => {
     // We need to wait for one microtask after injecting the HTML into to
     // controller so that the actions have been bound to the controller.
     await Promise.resolve()
-    instance.querySelector('button').click()
+    instance.querySelector('button')!.click()
     expect(instance.foo).to.have.callCount(1)
   })
 
@@ -277,7 +275,7 @@ describe('bind', () => {
     </bind-test-element>`)
     bind(instance)
     expect(instance.foo).to.have.callCount(0)
-    const el = instance.querySelector('div')
+    const el = instance.querySelector('div')!
     el.click()
     expect(instance.foo).to.have.callCount(1)
     el.setAttribute('data-action', 'click:other-element#foo')
@@ -292,7 +290,7 @@ describe('bind', () => {
     bind(instance)
     listenForBind(instance.ownerDocument)
     await Promise.resolve()
-    const button = instance.querySelector('button')
+    const button = instance.querySelector('button')!
     button.click()
     expect(instance.foo).to.have.callCount(0)
     button.setAttribute('data-action', 'click:bind-test-element#foo')
diff --git a/test/controller.ts b/test/controller.ts
index 232fede..cb6abe5 100644
--- a/test/controller.ts
+++ b/test/controller.ts
@@ -25,7 +25,6 @@ describe('controller', () => {
 
   it('binds controllers before custom connectedCallback behaviour', async () => {
     @controller
-    // eslint-disable-next-line @typescript-eslint/no-unused-vars
     class ControllerBindOrderElement extends HTMLElement {
       foo = fake()
     }
@@ -36,7 +35,7 @@ describe('controller', () => {
         this.dispatchEvent(new CustomEvent('loaded'))
       }
     }
-    instance = await fixture(html`
+    instance = await fixture<ControllerBindOrderElement>(html`
       <controller-bind-order>
         <controller-bind-order-sub data-action="loaded:controller-bind-order#foo" />
       </controller-bind-order>
@@ -46,36 +45,34 @@ describe('controller', () => {
 
   it('binds shadowRoots after connectedCallback behaviour', async () => {
     @controller
-    // eslint-disable-next-line @typescript-eslint/no-unused-vars
     class ControllerBindShadowElement extends HTMLElement {
       connectedCallback() {
         this.attachShadow({mode: 'open'})
         const button = document.createElement('button')
         button.setAttribute('data-action', 'click:controller-bind-shadow#foo')
-        this.shadowRoot.appendChild(button)
+        this.shadowRoot!.appendChild(button)
       }
 
       foo() {
         return 'foo'
       }
     }
-    instance = await fixture(html`<controller-bind-shadow />`)
+    instance = await fixture<ControllerBindShadowElement>(html`<controller-bind-shadow />`)
     replace(instance, 'foo', fake(instance.foo))
 
-    instance.shadowRoot.querySelector('button').click()
+    instance.shadowRoot!.querySelector('button')!.click()
 
     expect(instance.foo).to.have.callCount(1)
   })
 
   it('binds auto shadowRoots', async () => {
     @controller
-    // eslint-disable-next-line @typescript-eslint/no-unused-vars
     class ControllerBindAutoShadowElement extends HTMLElement {
       foo() {
         return 'foo'
       }
     }
-    instance = await fixture(html`
+    instance = await fixture<ControllerBindAutoShadowElement>(html`
       <controller-bind-auto-shadow>
         <template data-shadowroot="open">
           <button data-action="click:controller-bind-auto-shadow#foo" />
@@ -86,8 +83,8 @@ describe('controller', () => {
 
     expect(instance.shadowRoot).to.exist
     expect(instance).to.have.property('shadowRoot').not.equal(null)
-    expect(instance.shadowRoot.children).to.have.lengthOf(1)
-    instance.shadowRoot.querySelector('button').click()
+    expect(instance.shadowRoot!.children).to.have.lengthOf(1)
+    instance.shadowRoot!.querySelector('button')!.click()
 
     expect(instance.foo).to.have.callCount(1)
   })
@@ -97,15 +94,14 @@ describe('controller', () => {
     // eslint-disable-next-line @typescript-eslint/no-unused-vars
     class ChildElementElement extends HTMLElement {}
     @controller
-    // eslint-disable-next-line @typescript-eslint/no-unused-vars
     class ParentElementElement extends HTMLElement {
       connectedCallback() {
-        const child = this.querySelector('child-element')
+        const child = this.querySelector('child-element')!
         expect(child.matches(':defined')).to.equal(true)
       }
     }
 
-    instance = await fixture(html`
+    instance = await fixture<ParentElementElement>(html`
       <parent-element>
         <child-element />
       </parent-element>
@@ -113,12 +109,12 @@ describe('controller', () => {
   })
 
   describe('attrs', () => {
-    let attrValues = []
+    let attrValues: string[] = []
     @controller
     class AttributeTestElement extends HTMLElement {
       foo = 'baz'
       attributeChangedCallback() {
-        attrValues.push(this.getAttribute('data-foo'))
+        attrValues.push(this.getAttribute('data-foo')!)
         attrValues.push(this.foo)
       }
     }
@@ -129,14 +125,14 @@ describe('controller', () => {
     })
 
     it('initializes attrs as attributes in attributeChangedCallback', async () => {
-      instance = await fixture(html`<attribute-test></attribute-test>`)
+      instance = await fixture<AttributeTestElement>(html`<attribute-test></attribute-test>`)
       instance.foo = 'bar'
       instance.attributeChangedCallback()
       expect(attrValues).to.eql(['bar', 'bar'])
     })
 
     it('initializes attributes as attrs in attributeChangedCallback', async () => {
-      instance = await fixture(html`<attribute-test />`)
+      instance = await fixture<AttributeTestElement>(html`<attribute-test />`)
       instance.setAttribute('data-foo', 'bar')
       instance.attributeChangedCallback()
       expect(attrValues).to.eql(['bar', 'bar'])
diff --git a/test/dasherize.ts b/test/dasherize.ts
index fc4a839..0fbf0fb 100644
--- a/test/dasherize.ts
+++ b/test/dasherize.ts
@@ -2,7 +2,7 @@ import {expect} from '@open-wc/testing'
 import {dasherize} from '../src/dasherize.js'
 
 describe('dasherize', () => {
-  const tests = [
+  const tests: Array<[PropertyKey, string]> = [
     ['json', 'json'],
     ['fooBar', 'foo-bar'],
     ['FooBar', 'foo-bar'],
diff --git a/test/mark.ts b/test/mark.ts
index 266352d..b6b42e6 100644
--- a/test/mark.ts
+++ b/test/mark.ts
@@ -6,8 +6,8 @@ describe('createMark', () => {
   it('returns a tuple of functions: a mark and getMarks', () => {
     const mark = createMark(() => {})
     expect(mark).to.be.an('array').with.lengthOf(2)
-    expect(mark).to.have.property(0).a('function')
-    expect(mark).to.have.property(1).a('function')
+    expect(mark).to.have.property('0').a('function')
+    expect(mark).to.have.property('1').a('function')
   })
 
   it('attaches a `static` unique symbol to the first function', () => {
@@ -20,7 +20,7 @@ describe('createMark', () => {
   it('can be added to class fields without errors', () => {
     const [mark] = createMark(() => {})
     class FooBar {
-      @mark foo
+      @mark foo: unknown
       @mark bar = 1
       @mark baz = 'hi'
     }
@@ -30,11 +30,15 @@ describe('createMark', () => {
   it('can be added to getters or setters without errors', () => {
     const [mark] = createMark(() => {})
     class FooBar {
-      @mark get foo() {}
-      set foo(v) {}
+      @mark get foo() {
+        return 1
+      }
+      set foo(v: number) {}
 
-      @mark get bar() {}
-      @mark set baz(v) {}
+      @mark get bar() {
+        return 1
+      }
+      @mark set baz(v: number) {}
     }
     new FooBar()
   })
@@ -50,13 +54,17 @@ describe('createMark', () => {
   it('retrieves all marked fields with the get mark function', () => {
     const [mark, getMark] = createMark(() => {})
     class FooBar {
-      @mark foo
+      @mark foo: unknown
       @mark bar = 1
       @mark baz = 'hi'
-      @mark get bing() {}
-      @mark get qux() {}
-      @mark set quuz(v) {}
-      @mark set corge(v) {}
+      @mark get bing() {
+        return 1
+      }
+      @mark get qux() {
+        return 1
+      }
+      @mark set quuz(v: number) {}
+      @mark set corge(v: number) {}
       @mark grault() {}
     }
     expect(getMark(new FooBar())).to.eql(new Set(['foo', 'bar', 'baz', 'bing', 'qux', 'quuz', 'corge', 'grault']))
@@ -75,13 +83,17 @@ describe('createMark', () => {
     const [mark, getMark] = createMark(() => {})
     class FooBar {
       static [mark.static] = ['bar', 'bing', 'quuz', 'grault']
-      @mark foo
+      @mark foo: unknown
       bar = 1
       @mark baz = 'hi'
-      get bing() {}
-      @mark get qux() {}
-      set quuz(v) {}
-      @mark set corge(v) {}
+      get bing() {
+        return 1
+      }
+      @mark get qux() {
+        return 1
+      }
+      set quuz(v: number) {}
+      @mark set corge(v: number) {}
       grault() {}
     }
     const instance = new FooBar()
@@ -92,13 +104,17 @@ describe('createMark', () => {
     const [mark, getMark] = createMark(() => {})
     class FooBar {
       static [mark.static] = ['bar', 'bing', 'quuz', 'grault']
-      @mark foo
+      @mark foo: unknown
       @mark bar = 1
       @mark baz = 'hi'
-      @mark get bing() {}
-      @mark get qux() {}
-      @mark set quuz(v) {}
-      @mark set corge(v) {}
+      @mark get bing() {
+        return 1
+      }
+      @mark get qux() {
+        return 1
+      }
+      @mark set quuz(v: number) {}
+      @mark set corge(v: number) {}
       @mark grault() {}
     }
     expect(getMark(new FooBar())).to.eql(new Set(['foo', 'bar', 'baz', 'bing', 'qux', 'quuz', 'corge', 'grault']))
@@ -109,13 +125,17 @@ describe('createMark', () => {
     const [mark] = createMark(validate)
     const sym = Symbol('garply')
     class FooBar {
-      @mark foo
+      @mark foo: unknown
       @mark bar = 1
       @mark baz = 'hi'
-      @mark get bing() {}
-      @mark get qux() {}
-      @mark set quuz(v) {}
-      @mark set corge(v) {}
+      @mark get bing() {
+        return 1
+      }
+      @mark get qux() {
+        return 1
+      }
+      @mark set quuz(v: number) {}
+      @mark set corge(v: number) {}
       @mark grault() {}
       @mark [sym]() {}
     }
@@ -136,13 +156,17 @@ describe('createMark', () => {
     const [mark, getMark] = createMark(validate)
     class FooBar {
       static [mark.static] = ['foo', 'bar', 'baz', 'bing', 'qux', 'quuz', 'corge', 'grault']
-      foo
+      foo: unknown
       bar = 1
       baz = 'hi'
-      get bing() {}
-      get qux() {}
-      set quuz(v) {}
-      set corge(v) {}
+      get bing() {
+        return 1
+      }
+      get qux() {
+        return 1
+      }
+      set quuz(v: number) {}
+      set corge(v: number) {}
       grault() {}
     }
     getMark(new FooBar())
diff --git a/test/register.ts b/test/register.ts
index f39531e..7b923f9 100644
--- a/test/register.ts
+++ b/test/register.ts
@@ -9,19 +9,19 @@ describe('register', () => {
 
   it('registers the class as a custom element, normalising the class name', () => {
     @register
-    class MyFirstClass {}
+    class MyFirstClass extends HTMLElement {}
     expect(window.customElements.get('my-first-class')).to.equal(MyFirstClass)
   })
 
   it('does not register controllers that already exist', () => {
     {
       @register
-      class MySecondClass {}
+      class MySecondClass extends HTMLElement {}
       expect(window.customElements.get('my-second-class')).to.equal(MySecondClass)
     }
     {
       @register
-      class MySecondClass {}
+      class MySecondClass extends HTMLElement {}
       expect(window.customElements.get('my-second-class')).to.not.equal(MySecondClass)
     }
   })
@@ -31,23 +31,23 @@ describe('register', () => {
     replace(
       customElements,
       'get',
-      fake(() => class {})
+      fake(() => class extends HTMLElement {})
     )
     {
       @register
-      class MyThirdClass {}
+      class MyThirdClass extends HTMLElement {}
       expect(customElements.define).to.be.calledOnceWithExactly('my-third-class', MyThirdClass)
     }
     expect(() => {
       @register
-      class MyThirdClass {}
+      class MyThirdClass extends HTMLElement {}
       expect(customElements.define).to.be.calledOnceWithExactly('my-third-class', MyThirdClass)
     }).to.throw(Error)
   })
 
   it('dasherises class names', () => {
     @register
-    class ThisIsAnExampleOfDasherisedClassNames {}
+    class ThisIsAnExampleOfDasherisedClassNames extends HTMLElement {}
     expect(window.customElements.get('this-is-an-example-of-dasherised-class-names')).to.equal(
       ThisIsAnExampleOfDasherisedClassNames
     )
@@ -55,19 +55,19 @@ describe('register', () => {
 
   it('will intuitively dasherize acryonyms', () => {
     @register
-    class URLBar {}
+    class URLBar extends HTMLElement {}
     expect(window.customElements.get('url-bar')).to.equal(URLBar)
   })
 
   it('dasherizes cap suffixed names correctly', () => {
     @register
-    class ClipX {}
+    class ClipX extends HTMLElement {}
     expect(window.customElements.get('clip-x')).to.equal(ClipX)
   })
 
   it('automatically drops the `Element` suffix', () => {
     @register
-    class FirstSuffixElement {}
+    class FirstSuffixElement extends HTMLElement {}
     expect(window.customElements.get('first-suffix')).to.equal(FirstSuffixElement)
   })
 })
diff --git a/test/target.ts b/test/target.ts
index 784df16..8967040 100644
--- a/test/target.ts
+++ b/test/target.ts
@@ -6,20 +6,20 @@ describe('Targetable', () => {
   @controller
   // eslint-disable-next-line @typescript-eslint/no-unused-vars
   class TargetTestElement extends HTMLElement {
-    @target foo
+    @target foo!: Element
     bar = 'hello'
-    @target baz
-    @target qux
-    @target shadow
+    @target baz!: Element
+    @target qux!: Element
+    @target shadow!: Element
 
-    @target bing
-    @targets foos
+    @target bing!: Element
+    @targets foos!: Element[]
     bars = 'hello'
-    @target quxs
-    @target shadows
+    @target quxs!: Element[]
+    @target shadows!: Element[]
   }
 
-  let instance
+  let instance: HTMLElement
   beforeEach(async () => {
     instance = await fixture(html`<target-test>
       <target-test>
@@ -61,7 +61,7 @@ describe('Targetable', () => {
       instance.attachShadow({mode: 'open'})
       const el = document.createElement('div')
       el.setAttribute('data-target', 'target-test.shadow')
-      instance.shadowRoot.appendChild(el)
+      instance.shadowRoot!.appendChild(el)
       expect(instance).to.have.property('shadow', el)
     })
 
@@ -69,7 +69,7 @@ describe('Targetable', () => {
       instance.attachShadow({mode: 'open'})
       const shadowEl = document.createElement('div')
       shadowEl.setAttribute('data-target', 'target-test.foo')
-      instance.shadowRoot.appendChild(shadowEl)
+      instance.shadowRoot!.appendChild(shadowEl)
       expect(instance).to.have.property('foo', shadowEl)
     })
   })
@@ -85,7 +85,7 @@ describe('Targetable', () => {
       instance.attachShadow({mode: 'open'})
       const els = [document.createElement('div'), document.createElement('div'), document.createElement('div')]
       for (const el of els) el.setAttribute('data-targets', 'target-test.foos')
-      instance.shadowRoot.append(...els)
+      instance.shadowRoot!.append(...els)
 
       expect(instance).to.have.property('foos').with.lengthOf(5)
       expect(instance).to.have.nested.property('foos[0]', els[0])
diff --git a/tsconfig.build.json b/tsconfig.build.json
new file mode 100644
index 0000000..2ab0489
--- /dev/null
+++ b/tsconfig.build.json
@@ -0,0 +1,10 @@
+{
+  "exclude": ["test"],
+  "extends": "./tsconfig.json",
+  "compilerOptions": {
+    "declaration": true,
+    "declarationMap": true,
+    "outDir": "./lib",
+    "noEmit": false
+  }
+}
diff --git a/tsconfig.json b/tsconfig.json
index e9a43b5..6767646 100644
--- a/tsconfig.json
+++ b/tsconfig.json
@@ -1,16 +1,13 @@
 {
-  "include": ["src"],
+  "include": ["src", "test"],
   "compilerOptions": {
     "baseUrl": ".",
-    "declaration": true,
-    "declarationMap": true,
     "esModuleInterop": true,
     "experimentalDecorators": true,
     "lib": ["es6", "dom", "dom.iterable"],
     "module": "ESNext",
     "moduleResolution": "node",
-    "noEmit": false,
-    "outDir": "./lib",
+    "noEmit": true,
     "sourceMap": true,
     "strict": true,
     "target": "ES2020"