Rather than reporting every infinitesimal change in how much a target element is visible, the Intersection Observer API uses thresholds. When you create an observer, you can provide one or more numeric values representing percentages of the target element which are visible. Then, the API only reports changes to visibility which cross these thresholds. Rather than reporting every infinitesimal change in how much a target element is visible, the Intersection Observer API uses thresholds. When you create an observer, you can provide one or more numeric values representing percentages of the target element which are visible. Then, the API only reports changes to visibility which cross these thresholds.