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2. Canvas (basic support) (2014). http://caniuse.com/#feat=canvas
3. Introducing “NoCAPTCHA reCAPTCHA”, December 2014. http://googleonlinesecurity.blogspot.co.uk/2014/12/are-you-robot-introducing-no-captcha.html
4. Usage of server-side programming languages for websites (2014). http://w3techs.com/technologies/overview/programming_language/all/
5. recaptcha plugins, December 2015. https://developers.google.com/recaptcha