I have been developing some interactive SVG running under WEBrick. Works fine until the site goes under SSL. At that point my Javascript code fails to return a document using either contentDocument, or getSVGDocument(). I don't really understand the fix, but following a hint from a list back in 2007, everything works again when i put the following line in httputils.rb. On my installation that is at
/usr/lib/ruby/1.8/webrick/httputils.rb
"svg" => "image/svg+xml",
added into the object DefaultMimeTypes.
Here is the header to httputils.rb