How to make icons clear in iOS?

Question

css

Good day. There is a site with such a slider dg-education.com/chastnye-shkoly-v-shveycarija at the top of the page. The customer says that when viewed from Iphone \ Macbook and IOS in general, the icons are displayed blurry, although on Windows / Android it seems to be fine. I heard something about device-pixel-ratio, but I can't quite figure out how to use it. Who faced it? I would be grateful for any advice.

Answer 1

https://habrahabr.ru/post/139682/

Answer 2

Designer's Guide to DPI