Images should include dimension attributes. Without `width` and `height`
specified, image dimensions are 0×0 pixels at first. ... In this case the
browser determines that all of them are visible to the user and decides to load
everything [1].
In CSS the `width` is set to a value and the `height` is unsed to scale the image
proportional in both dimensions.
[1] https://web.dev/browser-level-image-lazy-loading/#images-should-include-dimension-attributes
[2] https://caniuse.com/loading-lazy-attr
Signed-off-by: Markus Heiser <markus.heiser@darmarit.de>