The WMS 1.1.1 GetCapabilities XML response uses the OGC specific mime type application/vnd.ogc.wms_xml. I would like to propose a new WMS vendor parameter named FORMAT that can be used to force the response mime type to something more well recognized like text/xml. Note that WMS 1.3.0 GetCapabilities result already uses text/xml.
This will imply implementing the handling of the FORMAT parameter when generating the GetCapabilities response for WMS 1.1.1 and adding text/xml as a supported mime type for WMS 1.1.1 GetCapabilities result. In the case were an invalid mime type is requested we will log an warning and fall-back on the OGC default one.
The documentation should also be updated.