[4] HTTP の状態符号 103 は、いくつかの異なる用法で用いられているようです。

[1] HTTP Status Codes (Dan McCormick 著, 2011-10-07 11:42:55 +09:00 版) http://www.mccormickfamily.com/become-an-author/56-http-status-codes#103

[2] ResumableHttpRequestsProposal - gears - A proposal for supporting resumable POST/PUT HTTP requests in HTTP/1.0. - Improving Your Web Browser - Google Project Hosting ( (2012-02-19 13:12:21 +09:00 版)) http://code.google.com/p/gears/wiki/ResumableHttpRequestsProposal

[3] draft-kazuho-early-hints-status-code-00 - An HTTP Status Code for Indicating Hints (2016-11-01 17:11:34 +09:00) https://tools.ietf.org/html/draft-kazuho-early-hints-status-code-00

[5] mod_http2 - Apache HTTP Server Version 2.4 ( (2017-01-15 21:40:17 +09:00)) https://httpd.apache.org/docs/2.4/mod/mod_http2.html#H2EarlyHints

[6] HTTP 103 Status · Issue #134 · mozilla/standards-positions (2019-02-07 16:05:30 +09:00) https://github.com/mozilla/standards-positions/issues/134

[7] 671310 - Support 103 Early Hints in HTTP/2. - chromium (2020-12-18T05:25:13.000Z) https://bugs.chromium.org/p/chromium/issues/detail?id=671310