Use Text.RichText and fontSizeMode: Text.Fit together?

[alexej.k]

I tried to create a Text element with rich text that auto-fits font size to fill its parent:

Rectangle {
           color: "white"
           width: 300
           height: 300

           Text {
                        text: "12<span style=\"font-weight:100\">3</span>"
                        fontSizeMode: Text.Fit                      
                        font.pointSize: 80
                        textFormat: Text.RichText
                        anchors.fill: parent
                        minimumPointSize: 20
           }
}

However, it only works when I remove textFormat: Text.RichText. Otherwise, the text is not scaled.
It works with textFormat: Text.StyledText, but then I can't use advanced formattings (I need the font weight to be changed).

Is it possible to achieve auto fitting with rich text somehow?