diff --git a/chrome/browser/resources/download_shelf/app.ts b/chrome/browser/resources/download_shelf/app.ts
index 1680034ec64f17..c370d495f33b7c 100644
--- a/chrome/browser/resources/download_shelf/app.ts
+++ b/chrome/browser/resources/download_shelf/app.ts
@@ -24,25 +24,22 @@ export class DownloadShelfAppElement extends CustomElement {
   constructor() {
     super();
 
-    /** @private {!DownloadShelfApiProxy} */
     this.apiProxy_ = DownloadShelfApiProxyImpl.getInstance();
 
-    const showAllButton = this.$('#show-all-button') as HTMLElement;
+    const showAllButton = this.$<HTMLElement>('#show-all-button')!;
     showAllButton.innerText = loadTimeData.getString('showAll');
     showAllButton.addEventListener('click', () => this.onShowAll_());
 
-    const closeButton = this.$('#close-button');
+    const closeButton = this.$('#close-button')!;
     closeButton.setAttribute('aria-label', loadTimeData.getString('close'));
     closeButton.addEventListener('click', () => this.onClose_());
   }
 
-  /** @private */
-  onShowAll_() {
+  private onShowAll_() {
     this.apiProxy_.doShowAll();
   }
 
-  /** @private */
-  onClose_() {
+  private onClose_() {
     this.apiProxy_.doClose();
   }
 }
diff --git a/chrome/browser/resources/download_shelf/download_item.ts b/chrome/browser/resources/download_shelf/download_item.ts
index d19f678ecde219..3003795739901f 100644
--- a/chrome/browser/resources/download_shelf/download_item.ts
+++ b/chrome/browser/resources/download_shelf/download_item.ts
@@ -44,20 +44,20 @@ export class DownloadItemElement extends CustomElement {
     this.opened = false;
     this.apiProxy_ = DownloadShelfApiProxyImpl.getInstance();
 
-    this.$('#shadow-mask')
-        .addEventListener('click', () => this.onOpenButtonClick_());
-    this.$('#dropdown-button')
-        .addEventListener('click', e => this.onDropdownButtonClick_(e));
+    this.$('#shadow-mask')!.addEventListener(
+        'click', () => this.onOpenButtonClick_());
+    this.$('#dropdown-button')!.addEventListener(
+        'click', e => this.onDropdownButtonClick_(e));
     const discardButton = this.$('#discard-button') as HTMLElement;
     discardButton.innerText = loadTimeData.getString('discardButtonText');
     discardButton.addEventListener('click', () => this.onDiscardButtonClick_());
-    this.$('#keep-button')
-        .addEventListener('click', () => this.onKeepButtonClick_());
+    this.$('#keep-button')!.addEventListener(
+        'click', () => this.onKeepButtonClick_());
     this.addEventListener('contextmenu', e => this.onContextMenu_(e));
 
-    this.$('.progress-indicator').addEventListener('animationend', () => {
-      this.$('.progress-indicator')
-          .classList.remove('download-complete-animation');
+    this.$('.progress-indicator')!.addEventListener('animationend', () => {
+      this.$('.progress-indicator')!.classList.remove(
+          'download-complete-animation');
     });
   }
 
@@ -132,8 +132,8 @@ export class DownloadItemElement extends CustomElement {
           this.progress = 1;
           // Only start animation if it's called from OnDownloadUpdated.
           if (this.downloadUpdated_) {
-            this.$('.progress-indicator')
-                .classList.add('download-complete-animation');
+            this.$('.progress-indicator')!.classList.add(
+                'download-complete-animation');
           }
           break;
         case DownloadState.kInterrupted:
diff --git a/chrome/browser/resources/download_shelf/download_list.ts b/chrome/browser/resources/download_shelf/download_list.ts
index 740ceeaf6bfd59..16266ac6561e69 100644
--- a/chrome/browser/resources/download_shelf/download_list.ts
+++ b/chrome/browser/resources/download_shelf/download_list.ts
@@ -31,7 +31,7 @@ export class DownloadListElement extends CustomElement {
     super();
 
     this.apiProxy_ = DownloadShelfApiProxyImpl.getInstance();
-    this.listElement_ = this.$('#download-list') as HTMLElement;
+    this.listElement_ = this.$<HTMLElement>('#download-list')!;
     this.resizeObserver_ = new ResizeObserver(() => this.updateElements_());
     this.resizeObserver_.observe(this.listElement_);
 
@@ -123,7 +123,7 @@ export class DownloadListElement extends CustomElement {
   private clear_() {
     while (this.listenerIds_.length) {
       this.apiProxy_.getCallbackRouter().removeListener(
-          this.listenerIds_.shift());
+          this.listenerIds_.shift()!);
     }
 
     while (this.listElement_.firstChild) {
diff --git a/chrome/browser/resources/download_shelf/tsconfig_base.json b/chrome/browser/resources/download_shelf/tsconfig_base.json
index 895f3746b7d473..3e71f76307c7b0 100644
--- a/chrome/browser/resources/download_shelf/tsconfig_base.json
+++ b/chrome/browser/resources/download_shelf/tsconfig_base.json
@@ -4,7 +4,6 @@
     "allowJs": true,
     "noUncheckedIndexedAccess": false,
     "noUnusedLocals": false,
-    "strictNullChecks": false,
     "strictPropertyInitialization": false
   }
 }
diff --git a/chrome/browser/resources/tab_strip/alert_indicators.ts b/chrome/browser/resources/tab_strip/alert_indicators.ts
index 5df1ad38831d57..e930c00de0ae2c 100644
--- a/chrome/browser/resources/tab_strip/alert_indicators.ts
+++ b/chrome/browser/resources/tab_strip/alert_indicators.ts
@@ -20,7 +20,7 @@ export class AlertIndicatorsElement extends CustomElement {
   constructor() {
     super();
 
-    this.containerEl_ = this.$('#container') as HTMLElement;
+    this.containerEl_ = this.$<HTMLElement>('#container')!;
 
     const audioIndicator = new AlertIndicatorElement();
     const recordingIndicator = new AlertIndicatorElement();
diff --git a/chrome/browser/resources/tab_strip/tab.ts b/chrome/browser/resources/tab_strip/tab.ts
index 7c6f7bd7c305f5..ffeb6724a8f5f2 100644
--- a/chrome/browser/resources/tab_strip/tab.ts
+++ b/chrome/browser/resources/tab_strip/tab.ts
@@ -61,29 +61,29 @@ export class TabElement extends CustomElement {
     super();
 
     this.alertIndicatorsEl_ =
-        this.$('tabstrip-alert-indicators') as AlertIndicatorsElement;
-    // Normally, custom elements will get upgraded automatically once added to
-    // the DOM, but TabElement may need to update properties on
+        this.$<AlertIndicatorsElement>('tabstrip-alert-indicators')!;
+    // Normally, custom elements will get upgraded automatically once added
+    // to the DOM, but TabElement may need to update properties on
     // AlertIndicatorElement before this happens, so upgrade it manually.
     customElements.upgrade(this.alertIndicatorsEl_);
 
-    this.closeButtonEl_ = this.$('#close') as HTMLElement;
+    this.closeButtonEl_ = this.$<HTMLElement>('#close')!;
     this.closeButtonEl_.setAttribute(
         'aria-label', loadTimeData.getString('closeTab'));
 
-    this.dragImageEl_ = this.$('#dragImage') as HTMLElement;
+    this.dragImageEl_ = this.$<HTMLElement>('#dragImage')!;
 
-    this.tabEl_ = this.$('#tab') as HTMLElement;
+    this.tabEl_ = this.$<HTMLElement>('#tab')!;
 
-    this.faviconEl_ = this.$('#favicon') as HTMLElement;
+    this.faviconEl_ = this.$<HTMLElement>('#favicon')!;
 
-    this.thumbnailContainer_ = this.$('#thumbnail') as HTMLElement;
+    this.thumbnailContainer_ = this.$<HTMLElement>('#thumbnail')!;
 
-    this.thumbnail_ = this.$('#thumbnailImg') as HTMLImageElement;
+    this.thumbnail_ = this.$<HTMLImageElement>('#thumbnailImg')!;
 
     this.tabsApi_ = TabsApiProxyImpl.getInstance();
 
-    this.titleTextEl_ = this.$('#titleText') as HTMLElement;
+    this.titleTextEl_ = this.$<HTMLElement>('#titleText')!;
 
     /**
      * Flag indicating if this TabElement can accept dragover events. This
diff --git a/chrome/browser/resources/tab_strip/tab_group.ts b/chrome/browser/resources/tab_strip/tab_group.ts
index bb0be405606676..c2a9314008b89e 100644
--- a/chrome/browser/resources/tab_strip/tab_group.ts
+++ b/chrome/browser/resources/tab_strip/tab_group.ts
@@ -22,7 +22,7 @@ export class TabGroupElement extends CustomElement {
 
     this.tabsApi_ = TabsApiProxyImpl.getInstance();
 
-    this.chip_ = this.$('#chip') as HTMLElement;
+    this.chip_ = this.$<HTMLElement>('#chip')!;
     this.chip_.addEventListener('click', () => this.onClickChip_());
     this.chip_.addEventListener(
         'keydown', e => this.onKeydownChip_(/** @type {!KeyboardEvent} */ (e)));
@@ -43,13 +43,13 @@ export class TabGroupElement extends CustomElement {
   }
 
   getDragImage(): HTMLElement {
-    return this.$('#dragImage') as HTMLElement;
+    return this.$<HTMLElement>('#dragImage')!;
   }
 
   getDragImageCenter(): HTMLElement {
     // Since the drag handle is #dragHandle, the drag image should be
     // centered relatively to it.
-    return this.$('#dragHandle') as HTMLElement;
+    return this.$<HTMLElement>('#dragHandle')!;
   }
 
   private onClickChip_() {
@@ -96,7 +96,7 @@ export class TabGroupElement extends CustomElement {
   }
 
   updateVisuals(visualData: TabGroupVisualData) {
-    (this.$('#title') as HTMLElement).innerText = visualData.title;
+    this.$<HTMLElement>('#title')!.innerText = visualData.title;
     this.style.setProperty('--tabstrip-tab-group-color-rgb', visualData.color);
     this.style.setProperty(
         '--tabstrip-tab-group-text-color-rgb', visualData.textColor);
diff --git a/chrome/browser/resources/tab_strip/tab_list.ts b/chrome/browser/resources/tab_strip/tab_list.ts
index 5dee17189aafb2..8487eb9d64ffae 100644
--- a/chrome/browser/resources/tab_strip/tab_list.ts
+++ b/chrome/browser/resources/tab_strip/tab_list.ts
@@ -388,13 +388,12 @@ export class TabListElement extends CustomElement implements
   }
 
   private findTabElement_(tabId: number): TabElement|null {
-    return this.$(`tabstrip-tab[data-tab-id="${tabId}"]`) as TabElement | null;
+    return this.$<TabElement>(`tabstrip-tab[data-tab-id="${tabId}"]`);
   }
 
   private findTabGroupElement_(groupId: string): TabGroupElement|null {
-    return this.$(`tabstrip-tab-group[data-group-id="${groupId}"]`) as
-        TabGroupElement |
-        null;
+    return this.$<TabGroupElement>(
+        `tabstrip-tab-group[data-group-id="${groupId}"]`);
   }
 
   private fetchAndUpdateColors_() {
@@ -403,8 +402,7 @@ export class TabListElement extends CustomElement implements
   }
 
   private fetchAndUpdateGroupData_() {
-    const tabGroupElements =
-        this.$all('tabstrip-tab-group') as NodeListOf<TabGroupElement>;
+    const tabGroupElements = this.$all<TabGroupElement>('tabstrip-tab-group');
     this.tabsApi_.getGroupVisualData().then(({data}) => {
       tabGroupElements.forEach(tabGroupElement => {
         tabGroupElement.updateVisuals(
@@ -420,7 +418,7 @@ export class TabListElement extends CustomElement implements
   }
 
   private getActiveTab_(): TabElement|null {
-    return this.$('tabstrip-tab[active]') as TabElement | null;
+    return this.$<TabElement>('tabstrip-tab[active]');
   }
 
   getIndexOfTab(tabElement: TabElement): number {
@@ -473,7 +471,7 @@ export class TabListElement extends CustomElement implements
     // document. When the tab strip first gains keyboard focus, no such event
     // exists yet, so the outline needs to be explicitly set to visible.
     this.focusOutlineManager_.visible = true;
-    (this.$('tabstrip-tab') as HTMLElement).focus();
+    this.$<TabElement>('tabstrip-tab')!.focus();
   }
 
   private onTabActivated_(tabId: number) {
@@ -488,7 +486,7 @@ export class TabListElement extends CustomElement implements
     // have updated a Tab to have an active state. For example, if a
     // tab is created with an already active state, there may be 2 active
     // TabElements: the newly created tab and the previously active tab.
-    (this.$all('tabstrip-tab[active]') as NodeListOf<TabElement>)
+    this.$all<TabElement>('tabstrip-tab[active]')
         .forEach((previouslyActiveTab) => {
           if (previouslyActiveTab.tab.id !== tabId) {
             previouslyActiveTab.tab = /** @type {!Tab} */ (
@@ -705,7 +703,7 @@ export class TabListElement extends CustomElement implements
       index++;
     }
 
-    let elementAtIndex = this.$all('tabstrip-tab')[index];
+    let elementAtIndex = this.$all('tabstrip-tab')[index]!;
     if (elementAtIndex && elementAtIndex.parentElement &&
         isTabGroupElement(elementAtIndex.parentElement)) {
       elementAtIndex = elementAtIndex.parentElement;
@@ -796,7 +794,8 @@ export class TabListElement extends CustomElement implements
           element, this.pinnedTabsElement_.childNodes[index]!);
     } else {
       let elementToInsert: TabElement|TabGroupElement = element;
-      let elementAtIndex = this.$all('tabstrip-tab').item(index);
+      let elementAtIndex: TabElement|TabGroupElement =
+          this.$all<TabElement>('tabstrip-tab').item(index);
       let parentElement = this.unpinnedTabsElement_;
 
       if (groupId) {
@@ -824,7 +823,7 @@ export class TabListElement extends CustomElement implements
         // TabElement is being sandwiched between two TabElements in a group, it
         // can be assumed that the tab will eventually be inserted into the
         // group as well.
-        elementAtIndex = elementAtIndex.parentElement;
+        elementAtIndex = elementAtIndex.parentElement as TabGroupElement;
       }
 
       if (elementAtIndex && elementAtIndex.parentElement === parentElement) {
diff --git a/ui/webui/resources/BUILD.gn b/ui/webui/resources/BUILD.gn
index 830ee0f736fa72..9d9dcab5be1a3d 100644
--- a/ui/webui/resources/BUILD.gn
+++ b/ui/webui/resources/BUILD.gn
@@ -215,6 +215,7 @@ ts_library("library") {
     "cr_elements/mwb_shared_style.ts",
     "cr_elements/mwb_shared_vars.ts",
     "cr_elements/search_highlight_style_css.ts",
+    "js/custom_element.ts",
     "js/i18n_mixin.ts",
     "js/list_property_update_mixin.ts",
     "js/web_ui_listener_mixin.ts",
@@ -260,7 +261,6 @@ ts_library("library") {
     "$root_dir/js/cr/ui/focus_outline_manager.m.d.ts",
     "$root_dir/js/cr/ui/focus_row.m.d.ts",
     "$root_dir/js/cr/ui/keyboard_shortcut_list.m.d.ts",
-    "$root_dir/js/custom_element.d.ts",
     "$root_dir/js/event_tracker.m.d.ts",
     "$root_dir/js/load_time_data.m.d.ts",
     "$root_dir/js/plural_string_proxy.d.ts",
@@ -323,7 +323,6 @@ ts_definitions("generate_definitions") {
     "js/cr/ui/focus_row.m.js",
     "js/cr/ui/keyboard_shortcut_list.m.js",
     "js/cr/ui/store.js",
-    "js/custom_element.js",
     "js/event_tracker.m.js",
     "js/icon.js",
     "js/load_time_data.m.js",
diff --git a/ui/webui/resources/js/BUILD.gn b/ui/webui/resources/js/BUILD.gn
index 8bb2f10b60c553..29fe46665db5ba 100644
--- a/ui/webui/resources/js/BUILD.gn
+++ b/ui/webui/resources/js/BUILD.gn
@@ -47,6 +47,7 @@ preprocess_if_expr("preprocess_src_ts") {
   in_folder = "./"
   out_folder = "$preprocess_folder"
   in_files = [
+    "custom_element.ts",
     "i18n_mixin.ts",
     "list_property_update_mixin.ts",
     "web_ui_listener_mixin.ts",
@@ -62,7 +63,6 @@ preprocess_if_expr("preprocess_src") {
     "assert.js",
     "color_utils.js",
     "cr.m.js",
-    "custom_element.js",
     "i18n_template_no_process.js",
     "icon.js",
     "load_time_data.m.js",
@@ -292,7 +292,6 @@ js_type_check("js_resources_modules") {
   deps = [
     ":assert.m",
     ":cr.m",
-    ":custom_element",
     ":event_tracker.m",
     ":i18n_behavior.m",
     ":icon",
@@ -328,9 +327,6 @@ js_library("cr.m") {
   externs_list = [ "$externs_path/chrome_send.js" ]
 }
 
-js_library("custom_element") {
-}
-
 js_library("event_tracker.m") {
   sources = [ "$root_gen_dir/ui/webui/resources/js/event_tracker.m.js" ]
   extra_deps = [ ":modulize_local" ]
diff --git a/ui/webui/resources/js/custom_element.js b/ui/webui/resources/js/custom_element.ts
similarity index 53%
rename from ui/webui/resources/js/custom_element.js
rename to ui/webui/resources/js/custom_element.ts
index 9d745688bb614d..b2f0f1a2d24081 100644
--- a/ui/webui/resources/js/custom_element.js
+++ b/ui/webui/resources/js/custom_element.ts
@@ -7,29 +7,27 @@
  * See the following file for usage:
  * chrome/test/data/webui/js/custom_element_test.js
  */
+
 export class CustomElement extends HTMLElement {
+  static get template(): string {
+    return '';
+  }
+
   constructor() {
     super();
 
     this.attachShadow({mode: 'open'});
     const template = document.createElement('template');
-    template.innerHTML = this.constructor.template || '';
-    this.shadowRoot.appendChild(template.content.cloneNode(true));
+    template.innerHTML =
+        (this.constructor as typeof CustomElement).template || '';
+    this.shadowRoot!.appendChild(template.content.cloneNode(true));
   }
 
-  /**
-   * @param {string} query
-   * @return {?Element}
-   */
-  $(query) {
-    return this.shadowRoot.querySelector(query);
+  $<E extends Element = Element>(query: string): E|null {
+    return this.shadowRoot!.querySelector<E>(query);
   }
 
-  /**
-   * @param {string} query
-   * @return {!NodeList<!Element>}
-   */
-  $all(query) {
-    return this.shadowRoot.querySelectorAll(query);
+  $all<E extends Element = Element>(query: string): NodeListOf<E> {
+    return this.shadowRoot!.querySelectorAll<E>(query);
   }
 }