diff --git a/inputfiles/idl/Resize Observer.widl b/inputfiles/idl/Resize Observer.widl
new file mode 100644
index 000000000..385cdfd31
--- /dev/null
+++ b/inputfiles/idl/Resize Observer.widl	
@@ -0,0 +1,39 @@
+enum ResizeObserverBoxOptions {
+    "border-box", "content-box", "device-pixel-content-box"
+};
+
+dictionary ResizeObserverOptions {
+    ResizeObserverBoxOptions box = "content-box";
+};
+
+[Exposed=(Window),
+ Constructor(ResizeObserverCallback callback)]
+interface ResizeObserver {
+    void observe(Element target, optional ResizeObserverOptions options);
+    void unobserve(Element target);
+    void disconnect();
+};
+
+callback ResizeObserverCallback = void (FrozenArray<ResizeObserverEntry> entries, ResizeObserver observer);
+
+[Exposed=Window]
+interface ResizeObserverEntry {
+    readonly attribute Element target;
+    readonly attribute DOMRectReadOnly contentRect;
+    readonly attribute sequence<ResizeObserverSize> borderBoxSize;
+    readonly attribute sequence<ResizeObserverSize> contentBoxSize;
+    readonly attribute sequence<ResizeObserverSize> devicePixelContentBoxSize;
+};
+
+interface ResizeObserverSize {
+    readonly attribute unrestricted double inlineSize;
+    readonly attribute unrestricted double blockSize;
+};
+
+[Constructor(Element target)
+]
+interface ResizeObservation {
+    readonly attribute Element target;
+    readonly attribute ResizeObserverBoxOptions observedBox;
+    readonly attribute sequence<ResizeObserverSize> lastReportedSizes;
+};
diff --git a/inputfiles/idlSources.json b/inputfiles/idlSources.json
index da35b2b38..faed73457 100644
--- a/inputfiles/idlSources.json
+++ b/inputfiles/idlSources.json
@@ -430,6 +430,10 @@
         "url": "https://www.w3.org/TR/referrer-policy/",
         "title": "Referrer Policy"
     },
+    {
+        "url": "https://www.w3.org/TR/resize-observer/",
+        "title": "Resize Observer"
+    },
     {
         "url": "https://w3c.github.io/resource-timing/",
         "title": "Resource Timing"