Our user research shows that many of our users workaround slow internet connections by saving webpages and sharing them with each other via Whatsapp.
We lazy load images on Wikipedia to save our users data.
Users typically print by using the browser print method.
When printing an article, any lazy loaded images are not present in the printed version of the page.
Although it is not possible to download images during the print dialog, we can notify the user when images are not present in the article via a banner at the top of the page. The banner would be added on page load, positioned at the top of the page, but only displayed on print media. When all images are downloaded this banner would be removed.
Medium does this like so:
Note: Although not widely supported, architecture wise this should be expected to work on the Vector skin when useformat=mobile is applied given lazy loading is a "m. domain" concept, not a Minerva concept.
To be completed by @alexhollender