The HTTP 100 Continue informational status response code indicates that everything so far is OK and that the client should continue with the request or ignore it if it is already finished.
To have a server check the request's headers, a client must send Expect: 100-continue as a header in its initial request and receive a 100 Continue status code in response before sending the body.
100 Continue
| Specification | Title |
|---|---|
| RFC 7231, section 6.2.1: 100 Continue | Hypertext Transfer Protocol (HTTP/1.1): Semantics and Content |
| Desktop | ||||||
|---|---|---|---|---|---|---|
| Chrome | Edge | Firefox | Internet Explorer | Opera | Safari | |
| Basic support | Yes | Yes | Yes | Yes | Yes | Yes |
| Mobile | |||||||
|---|---|---|---|---|---|---|---|
| Android webview | Chrome for Android | Edge Mobile | Firefox for Android | Opera for Android | iOS Safari | Samsung Internet | |
| Basic support | Yes | Yes | Yes | Yes | Yes | Yes | Yes |
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https://developer.mozilla.org/en-US/docs/Web/HTTP/Status/100