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Protocol Handlers

The mailto: URL scheme is used to create links that should open the user's email client with a prepopulated email address as the receiver. Some mail clients also support passing CC, BCC, subject, and/or body. [MDN, RFC 6068]

The tel URL scheme is used to open a (phone) caller application with the specified phone number [RFC 3966, css-tricks.com]. Not all browsers and/or operating systems support this scheme, and there is currently no (reliable) way to feature detect it [stackoverflow.com].

Some operating systems may support more protocol handlers [developer-mdn.apple.com].

Custom protocol handlers can be defined with registerProtocolHandler() [MDN].