Incident documentation: https://wikitech.wikimedia.org/wiki/Incident_documentation/20161013-GlobalSign
Appears that MacOS users on Sierra are having certificate errors when using Safari and Chrome
Also appears to affect Microsoft Edge, and IE on Windows 10
The problem is caused by GlobalSign and has been reported here: https://twitter.com/globalsign/status/786505261842247680
If you're looking for an immediate workaround:
MacOS Sierra: install and use Firefox rather than Safari or Chrome
Win10: Opera 12, Firefox
Firefox can be downloaded from https://www.firefox.com/