Apart from the suggestions made in How can I see what my web site looks like in various browsers?, I have actively downloaded the 3 major browsers (Chrome, Firefox and Internet Explorer) and I check my website designs using the actual browsers along with Edge which is now provided automatically in Windows 10.
For belt and braces, I also double check mobile screen media queries and accessibility on my Samsung mobile phone and iPad.
To check percentage compatibility for global users I often use caniuse.com to get an idea on what percentage of web users globally will be able to use the CSS/HTML5/JS function.
Now even though I know it is highly accessible, out of interest, I went to see how accesible the CSS property
z-index is and I got the following information.
Unfortunately this is one of the rare situations where you don't get a global percentage score like you do with CSS
content-visibility (see: https://caniuse.com/css-content-visibility) but looking at the individual browsers, "Android Browser" is given.
What is "Android Browser"? Should we really be worried about it's lack of compatibility with
z-index when it seems that for 7 years at least, "Android Browser" has been dropped in favour of Google Chrome?