Supporting POST authentication/authorization requests is not mandatory for clients, but would be a nice addition if I wanted to use idtokenhint without being it captured in server logs in the referer header response.

>Authorization Servers MUST support the use of the HTTP GET and POST methods defined in RFC 2616 [RFC2616] at the Authorization Endpoint. Clients MAY use the HTTP GET or POST methods to send the Authorization Request to the Authorization Server. If using the HTTP GET method, the request parameters are serialized using URI Query String Serialization, per Section 13.1. If using the HTTP POST method, the request parameters are serialized using Form Serialization, per Section 13.2.

http://openid.net/specs/openid-connect-core-1_0.html