Lazy-loading placeholder (lazysizes) HTML

<!-- The empty GIF keeps src valid until lazysizes swaps in data-src.
     (For most pages today, native loading="lazy" is enough.) -->
<img src="data:image/gif;base64,R0lGODlhAQABAIAAAP///wAAACH5BAEAAAAALAAAAAABAAEAAAICRAEAOw=="
     data-src="/images/photo.jpg" class="lazyload"
     width="800" height="600" alt="A photo">