SpeedTest is an HTML5 app that lets you test websites at a variety of resolutions very quickly. It also works on mobile, which allows you to test websites at a variety of widths on a phone or tablet where you can't normally change the width of the browser.

Screen information such viewport & aspect ratio

To get the device's logical resolution/获取设备逻辑分辨率

Mandelbrot visualizer in under 1kb of JS and HTML

This is useful for checking width and height of the screen created in pure JS

#100DaysofCode | MDN skill assessment - Mozilla Splash Page Project (HTML | Responsive Images)

A super simple JavaScript application that tells you the screen resolution of your device.

Images Resizer Online allows you to change the dimensions or size of images through the web browsers without the need for software or applications.

Scale design dimensions (width and height) up or down for @1x, @2, and @3x resolutions.

Easily retrieve the width and height of your browser viewport