"This page could not be shown" at tablet or mobile size

The Tablet (768px) and Mobile (390px) buttons load the same page inside a frame. Some sites send a header (X-Frame-Options or a Content Security Policy with frame-ancestors) that forbids being shown in a frame. The preview then says the page could not be shown at that size.

To review that size anyway, make your browser window narrower. Comments are marked with the size worked out from the window’s width, so they are filed the same way.

If the site is yours, you can allow the site to frame itself (for example frame-ancestors 'self'), and the previews will work. See Check tablet and mobile sizes.