title: Which browsers send the expect: 100-continue header?
body: <p>The HTTP/1.1 spec allows for clients to send an <a href="https://stackoverflow.com/questions/5366663/what-does-100-continue-mean-in-the-put-request"><code>Expect: 100-continue</code></a> header for large request payloads (<a href="https://www.rfc-editor.org/rfc/rfc2616#section-8.2.3" rel="nofollow noreferrer"><em>8.2.3 Use of the 100 (Continue) Status - Hypertext Transfer Protocol -- HTTP/1.1 - RFC2616</em></a>).</p>
  <p>This will pause the request after the headers have been sent and allow the server to reject it based on those headers.</p>
  <p>If those headers did not indicate a bad request (e.g. a too large <code>Content-Length</code>), then it responds with a <code>100 Continue</code> status line, allowing the client to proceed. At this point the client sends the request body.</p>
  <p>Which browsers actually support this, and under which conditions will they send the expect header?</p>