Since I develop Joomla extensions only for my own use, and then make them available if it seems that others might find them useful, I only test compatibility with Browsers and Server configurations that I actually use.
Currently I am only mostly using the Edge browser on Linux (Ubuntu 20.0.4) and Android mobile devices - so I make no attempt to check how things look with other browsers - if something looks wrong to you then by all means send me details and I'll look into correcting it dreckly.
Similarly all websites I manage are running on Ubuntu 20.0.4 and using Apache, MariaDB, and PHP 7.4. For Joomla extensions I am only bothering with Joomla 3.9/10 and making no attempt to check earlier versions and have no plans to switch to Joomla 4 anytime soon. In fact I currently anticipate never switching any existing site to J4 (what would be the point - if it ain't broke don't fix it) and will only use J4 for any new site if someone pays me to.