Since version WebKitGTK version 2.2, developers can register a authenticate callback to the WebView. This gives a WebKitAuthenticationRequest which gives a chance to provide certificates with webkit_authentication_request_authenticate. While developing, it turned out that the WebSocket implementation does not handle client certificates.
PR: https://github.com/WebKit/WebKit/pull/6245
Committed 256780@main (7d61be00ed87): <https://commits.webkit.org/256780@main> Reviewed commits have been landed. Closing PR #6245 and removing active labels.