Be aware, that these days, visitors of websites does not use desktop device only, when browsing any website.
Currently, there exists at least 5 different types of devices, whose owners should enjoy visit of any website properly. These requires to verify, that each device displays website properly, no matter which device is currently being used. Just consider a situation, when contact details would not be aside of the display, so website visitor would not be possible to call or send a message. Another issue would be not properly made navigation menu - currently called as a hamburger menu. Easily and simple - no way how to navigate within the website - visitor leaves the web, and try to find your services at your competitor, who can have more user friendly website, that is optimized also for responsivity.
Also, these days, it is quite good habbit to place on each page Back to top button, not to make website visitor to scroll to top menu manualy, which can be kinda anoying. Just a single click to such button makes you again at the navigation menu, where You were at the moment, you started the reading of such website. Even such details can lead your page visitors to simply leave your website, because the navigation was not simply enough for them.
So currently, these are 5 major devices, that requires perfect testing and verification, that whole content is perfectly visible and working:
Usualy supports Full-HD resolution 1920×1080 7.88% of devices
Usualy supports the resolution 1440×900 – 2.68% of devices
Usualy supports the 375×667 – 4.42% of devices
Usualy supports the 1768×367 – 8.77%
Usualy supports the 1440×900 – 2.68%
The mask icon color have to be declared. Also the mask icon for Safari pinned tabs is present
Favicon.ico must be in the root of your web site in recommended sizes (16x16, 32x32, 48x48).