doc/srcset/iframe.html
<!DOCTYPE html>
<html>
<head></head>
<style>
html, body {
width: 100%;
height: 100%;
text-align: center;
overflow: none;
margin: 0;
padding: 0;
}
img {
max-width: 100%;
height: auto;
}
</style>
<body>
<img src="http://via.placeholder.com/1200x100" srcset="
http://via.placeholder.com/1600x100 1600w,
http://via.placeholder.com/1200x100 1200w,
http://via.placeholder.com/800x100 800w,
http://via.placeholder.com/640x100 640w,
http://via.placeholder.com/360x100 360w,
http://via.placeholder.com/200x100 200w,
" sizes="100vw" alt="" class="img-fluid img-responsive">
</body>
</html>