Now a days HTML5 responsive designs are very popular one and developers have more concern on developing websites and web based applications to be mobile compatibility. Developers need to simulate mobile responsibility for that many used to narrow down the browser width to check responsibility. Chrome come out with awesome feature in its browser to solver this issue.
In webpage you need to check, right click in any place on the website and click on inspect element or you can press ctrl+shift+i browser view will be changed as bellow
Then click on toggle device in toolbar or else simple press ctrl+shift+M. Your window will be changed as below screenshot
You can select pre-built in device simulation to test your application