Test a webpage for Retina display

To test retina display, just adjust the screen pixel ratio.

On Firefox

1. Go to the  Firefox advance configuration. (Type about:config on address bar)
2. Find layout.css.devPixelsPerPx
3. Set ratio to 1.5, 2.0 .... (1 is normal, -1.0 is default value)
Firefox

Default : layout.css.devPixelsPerPx = -1.0

On Chrome

1. F12 to open developer tools
2. Enable mobile emulation (Ctrl + Shift + M or Click on Mobile/Tablet icon)
3. At the top right of the page, click on menu icon (Three small points ⋮ ) Then "Add device pixel ration".
4. Now you can see a new dropdown menu "DPR:....", Set Device type to Responsive to change manually