This works for us in and international external collaborators in the XMML community,
once the site certificate has been installed with IE, using XMLSpy.
Attempting to validate with StylusStudio gets:
"FATAL ERROR: Cannot open https://www.seegrid.csiro.au/subversion/xmml/trunk/XMML/borehole.xsd "
... (Warning) schema_reference.4: Failed to read schema document
depending on the validator being used.
When I try to directly open a file like https://www.seegrid.csiro.au/subversion/xmml/trunk/XMML/borehole.xsd,
I can do so in IE and FireFox as well as XMLSpy.
However, StylusStudio doesn't seem able to.
Whether I have the My Computer or WebDav Server button selected, the file open returns
"Encountered an I/O error in URL ... an SSl-enabled server port"
I can't justify any more time experimenting, can't see anything
in the help or these forums related to these issues so hopefully someone can come up with an answer.