Most common: http: https: ftp: file: mailto: tel: sms: skype: data: irc: ... URI schemes should be registered with the Internet Assigned Numbers Authority (IANA), although non-registered schemes are used in practice.
Example of a link on my page that uses the web+tweet scheme. The “data“ populates new tweet. a href=“web+tweet:Hello%20Treehouse“>Tweet 'Hello Treehouse'< /a Clicking this link would trigger a GET request to: https://mytwitterapp.com/tweet?data=web%2Btweet%3AHello%2520Treehouse
Example explains that registerProtocolHandler allows Alice to do POST on Bob's page. So Alice can e.g. subscribe to Bob just by visiting Bob's page, without remembering details, URLs, etc
Explains how the PWA “protocol_handlers“ manifest declaration is related to the (very similar) registerProtocolHandler
Chapters 3.3 and 3.4. of Scheme book are about continuations
