Using Safari Tech Preview Release 51 (Safari 11.2, WebKit 13606.1.6) I'm visiting many site which requires SSL Client Certificate Auth. Safari is configured to remember which certificate it should use for my usual websites, so it don't have to ask each time I'm accessing them. And many times a day, when I try to access one of my usual site, I got a "HTTP 400 Bad Request: No required SSL certificate was sent". Reloading the page has not effect, but a simple and very effective workaround is to kill the networking process (pkill com.apple.WebKit.Networking). After killing it, reloading the page works as expected.
Created attachment 335588 [details] Console log of a load failure In case it may help, this is a log from the Console of a page load failure.
<rdar://problem/38531939>
The fix for this issue was needed outside the WebKit project, therefore this is being resolved as 'Moved'. The relevant component (in <rdar://problem/38531939>) could not make progress without a sysdiagnose after the event. If this is still occurring, could you please use Feedback Assistant to file a radar? This will capture relevant logging that would help resolve the issue.