See microsoft#885 (comment)

Author: jrandolf

baselines/dom.generated.d.ts

@@ -6615,6 +6615,7 @@ interface HTMLElement extends Element, DocumentAndElementEventHandlers, ElementC
     readonly offsetParent: Element | null;
     readonly offsetTop: number;
     readonly offsetWidth: number;
+    readonly parentElement: HTMLElement | undefined;
     spellcheck: boolean;
     title: string;
     translate: boolean;
@@ -13001,7 +13002,9 @@ interface SVGElement extends Element, DocumentAndElementEventHandlers, DocumentA
     /** @deprecated */
     readonly className: any;
     readonly ownerSVGElement: SVGSVGElement | null;
+    readonly parentElement: SVGElement | null;
     readonly viewportElement: SVGElement | null;
+    getElementsByClassName<E extends Element = SVGElement>(classNames: string): HTMLCollectionOf<E>;
     addEventListener<K extends keyof SVGElementEventMap>(type: K, listener: (this: SVGElement, ev: SVGElementEventMap[K]) => any, options?: boolean | AddEventListenerOptions): void;
     addEventListener(type: string, listener: EventListenerOrEventListenerObject, options?: boolean | AddEventListenerOptions): void;
     removeEventListener<K extends keyof SVGElementEventMap>(type: K, listener: (this: SVGElement, ev: SVGElementEventMap[K]) => any, options?: boolean | EventListenerOptions): void;
@@ -13589,6 +13592,7 @@ interface SVGForeignObjectElement extends SVGGraphicsElement {
     readonly width: SVGAnimatedLength;
     readonly x: SVGAnimatedLength;
     readonly y: SVGAnimatedLength;
+    getElementsByClassName<E extends Element = HTMLElement>(classNames: string): HTMLCollectionOf<E>;
     addEventListener<K extends keyof SVGElementEventMap>(type: K, listener: (this: SVGForeignObjectElement, ev: SVGElementEventMap[K]) => any, options?: boolean | AddEventListenerOptions): void;
     addEventListener(type: string, listener: EventListenerOrEventListenerObject, options?: boolean | AddEventListenerOptions): void;
     removeEventListener<K extends keyof SVGElementEventMap>(type: K, listener: (this: SVGForeignObjectElement, ev: SVGElementEventMap[K]) => any, options?: boolean | EventListenerOptions): void;

inputfiles/idl/DOM.widl

@@ -214,7 +214,7 @@ interface Node : EventTarget {
   readonly attribute Document? ownerDocument;
   Node getRootNode(optional GetRootNodeOptions options = {});
   readonly attribute Node? parentNode;
-  readonly attribute Element? parentElement;
+  readonly attribute HTMLElement? parentElement;
   boolean hasChildNodes();
   [SameObject] readonly attribute NodeList childNodes;
   readonly attribute Node? firstChild;

inputfiles/overridingTypes.json

@@ -510,6 +510,18 @@
                 },
                 "override-index-signatures": []
             },
+            "HTMLElement": {
+                "name": "HTMLElement",
+                "properties": {
+                    "property": {
+                        "parentElement": {
+                            "name": "parentElement",
+                            "read-only": 1,
+                            "override-type": "HTMLElement | undefined"
+                        }
+                    }
+                }
+            },
             "Document": {
                 "name": "Document",
                 "methods": {
@@ -909,6 +921,21 @@
                             "deprecated": 1,
                             "name": "className",
                             "type": "any"
+                        },
+                        "parentElement": {
+                            "name": "parentElement",
+                            "read-only": 1,
+                            "override-type": "SVGElement | null"
+                        }
+                    }
+                },
+                "methods": {
+                    "method": {
+                        "getElementsByClassName": {
+                            "name": "getElementsByClassName",
+                            "override-signatures": [
+                                "getElementsByClassName<E extends Element = SVGElement>(classNames: string): HTMLCollectionOf<E>"
+                            ]
                         }
                     }
                 }
@@ -954,6 +981,19 @@
                     }
                 }
             },
+            "SVGForeignObjectElement": {
+                "name": "SVGForeignObjectElement",
+                "methods": {
+                    "method": {
+                        "getElementsByClassName": {
+                            "name": "getElementsByClassName",
+                            "override-signatures": [
+                                "getElementsByClassName<E extends Element = HTMLElement>(classNames: string): HTMLCollectionOf<E>"
+                            ]
+                        }
+                    }
+                }
+            },
             "FormData": {
                 "name": "FormData",
                 "methods": {