The following description (which is introduced in the commit 0283e9e) in the example for loading
content attribute seems incorrect, that the width/height content attributes and CSS properties are swapped:
Developers are encouraged to specify an intrinsic aspect ratio via width and height attributes on lazy loaded images, even if CSS sets the image's width and height properties, to prevent the page layout from shifting around after the image loads.
(CSS properties do establish these dimensions.)