Media-types and their semantic were always supposed to be sideband information, just as the meaning of <a> and <form> is documented in HTML. An easy way to thi

Media-types and their semantic were always supposed to be sideband information, just as the meaning of <a> and <form> is documented in HTML. An easy way to think of it is that a Web Browser is a client for a REST architecture consisting of the web. If you create a new system and use a REST architectural style, the client app(s) will be the "browsers" for your API. When you implement a web browser, you do need to receive sideband information about the media-type (HTML) and your client application has to be able to understand those, including what <a> and <form> mean, for example. In a custom API, you use the same HTTP verbs if you want (REST doesn't mandate HTTP) but you need to define your media-types.