title: How to check if the OS handles certain URL protocols? (cross-browser)
body: <p>I have set my Cocoa app to be an URL handler for <code>com_example_myapp:</code> protocol (using <a href="https://stackoverflow.com/questions/49510/how-do-you-set-your-cocoa-application-as-the-default-web-browser">this great tutorial</a>). So, the link in Safari <code>&lt;a href="com_example_myapp://link"&gt;</code> now opens my app. Cool.</p>
  
  <p>But before I give the user the link in the web browser, how can I determine whether the client's OS is capable to handle it? How to check this via JS (PHP, ...)?</p>
  
  <p>I have found a lot of solutions (mostly for Mobile Safari) but cannot find anything cross-browser compatible.</p>