What they’re trying to accomplish here is “responsive web design” where the site looks similar across multiple devices.
It can be tricky because every element has to be styled to be responsive for it to work.
If they google responsive image galleries, there are a number of sites that will offer code snippets to help create layouts that will work well on a number of different screens.
In this case, if you style the
<hr> tags on the style sheet with some relative sizing, it could get them what they were after.
I used this code on the style sheet and think I have it closer to what they want …
Give it a shot and see if that helps get you closer. If they want to read more about relative sizing and about the vw (viewport window) here’s a link: https://www.w3schools.com/CSSref/css_units.asp
Hope this helps!