The intersectionRect read-only property of the PerformanceElementTiming interface returns the rectangle of the element within the viewport.

A DOMRectReadOnly which is the rectangle of the element within the viewport.

For display images this is the display rectangle of the image within the viewport. For text this is the display rectangle of the node in the viewport. This being the smallest rectangle that contains the union of all text nodes that belong to the element.

In this example an <img> element is being observed by adding the elementtiming attribute. A PerformanceObserver is registered to get all performance entries of type "element" and the buffered flag is used to access data from before observer creation. Calling entry.intersectionRect returns a DOMRectReadOnly object with the display rectangle of the image.

<img
  src="image.jpg"
  alt="a nice image"
  elementtiming="big-image"
  id="myImage" />

const observer = new PerformanceObserver((list) => {
  list.getEntries().forEach((entry) => {
    if (entry.identifier === "big-image") {
      console.log(entry.intersectionRect);
    }
  });
});
observer.observe({ type: "element", buffered: true });