He uses small background images that are places with CSS. Then, the classNames for the form elements are swithced with JavaScript. Marko also uses JavaScript to preloaded the custom images.

Add a short CSS class to an HTML Web page to use animated and stationary GIF images as the page's background. Open an HTML document and find the document's head section.

John Mueller said on Twitter "last I checked, we don't use CSS background images for image search." John said this also back in January 2018 when he said Google Image search does not index and ...