Responsive Web Design Tester isn’t a website, it’s a browser extension. You’ll find it in your browser’s extension or add-on library. The link below is for the Chrome extension but the tool works for Safari and Firefox as well. When you click on the extension icon you can choose from a list of presets and then the site opens in a new window at the viewport size you selected. The presets are a bit outdated but you’re able to add new devices and create your own preferred groups.