While browsers like Chrome and Firefox normally render very well on-screen any Web page, it is enough to give them the "Print" command for that page, to see in most cases a badly rendered page, where the size ratios of various elements are bad and they overlap or are mis-positioned so that the "printed" page is completely unlike what the browser shows on screen.

The way how the "printed" pages look in Firefox and Chrome demonstrates the same rendering problems that appear in most EPUB readers.

I have no idea which is the cause of this, but the bad behavior of "printing" in Firefox and Chrome has existed for years. Not all browsers behave the same, e.g. Vivaldi usually is much better at generating "printed" pages, than Chrome, despite being derived from the same code base.

Perhaps the great differences between on-screen rendering and "printed" rendering is caused by the fact that badly designed HTML/CSS might specify some sizes in "pixels" or other such inappropriate units, instead of using length units, like points, inches or millimeters. Then when rendering on different media the size ratios are corrupted.