While the applications are indeed useful for testing various responsive design aspects for your site, users are advised to seek other means, such as actual devices, for testing their web projects. After all, the sites listed above only simulate how a website is displayed. Actually running it on physical devices is a whole new ball game.
Some content simply cannot be displayed by using responsive design tools: for example, the way in which different high-performance devices, various browsers, or multi-touch screen control effects a website’s user experience. What’s more, the display of a website may be partly distorted when such tools are used. Scrollbars may appear during a responsive-design test, a feature that doesn’t exist for mobile devices with touch screen navigation.
Despite this, online tools prove to be valuable components for comprehensive responsive tests. Web designers especially benefit from the uncomplicated and quick modification of the content’s resolution; a feature that immensely accelerates the testing process.