mydevice.io is a useful website which reports the virtual CSS dimensions (including the CSS pixel ratio and dpi, dppx and dpcm resolutions) for the device and browser in a nice layout. mydevice.io also includes a table of those dimensions for common mobile devices.
Support Details was originally built to help provide tech support staff quickly figure out what type of computer and browser a user is using. Browser information can also be helpful to web designers.
The screen object contains information about the visitor's screen.
On an iPad, but not on other devices, screen.availWidth/availHeight seem to be flipped from screen.width/height.
The window object represents an open window in a browser. If a document contain frames (<iframe> tags), the browser creates one window object for the HTML document, and one additional window object for each frame.
The meaning of the values reported in the table below also vary from browser to browser and what mode the browser is operating in (quirks mode). Whether or not scrollbar widths are included may depend on the browser version and vintage.
On a mobile device without included in the webpage, window.innerWidth will report the full width of the page which can extend beyond the visible browser window, while window.outerWidth will report the width of just the browser window.
Finding well written and up-to-date articles which represent current browser standards is difficult. The following articles are interesting to read, but contain some information which may have been correct at the time they were, but now may have some incorrect information.
This Web Page
appCodeName (Mozilla), appName (Netscape) and product (Gecko) always seem to give the same answer, regardless of the browser being used.