<img src="https://www.gstatic.com/webp/gallery/1.webp" onerror="this.onerror=null; this.src='https://www.gstatic.com/webp/gallery/2.jpg'">
<div class="demo"></div>
.demo{
max-width : 400px;
height: 400px;
background-image:image("https://www.gstatic.com/webp/gallery/1.webp", "https://www.gstatic.com/webp/gallery/2.jpg")
}
Also see: Tab Triggers