!!! Overview [1]
[{$pagename}] is an [HTTP Status Code] indicating that everything so far is OK and that the client should continue with the [HTTP Request] or ignore it if it is already finished.

To have a server check the [HTTP Request Header]s, a client [MUST] send Expect: 100-continue as a header in its initial [HTTP Request] and receive a [{$pagename}] Continue status code in [HTTP Response] before sending the [HTTP Body].

!! More Information
There might be more information for this subject on one of the following:
[{ReferringPagesPlugin before='*' after='\n' }]
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* [#1] - [101 Switching Protocols|https://developer.mozilla.org/en-US/docs/Web/HTTP/Status/101|target=_blank]