What are some alternatives?
When comparing Responsa and Blisk, you can also consider the following products
BrowserPage - The most intuitive Responsive Design Checker online tool for your Website.
BrowserStack - Test your website for cross browser compatibility on real browsers. Instant access to multiple desktop and mobile browsers. Get Free Trial.
Polypane - The browser for developing responsive & accessible websites.
Browsershots - Browsershots makes screenshots of your web design in different browsers.