code: def _required_envs(env_vars):
      
      envs = {
          key: os.environ.get(key)
          for key in env_vars
      }
  
      # Check for missing keys
      missing = [key for key, val in list(envs.items()) if val is None]
      if missing:
          msg = (
              u"These environment variables must be set: " + u", ".join(missing)
          )
          raise BrowserConfigError(msg)
  
      # Check that we support this browser
      if envs['SELENIUM_BROWSER'] not in BROWSERS:
          msg = u"Unsuppported browser: {0}".format(envs['SELENIUM_BROWSER'])
          raise BrowserConfigError(msg)
  
      return envs
docs: Parse environment variables for required values,
      raising a `BrowserConfig` error if they are not found.
  
      Returns a `dict` of environment variables.