CSS: stroke-offset does not accept expressions

Hello,
I was trying to create a style visualising traffic flows as stroke width on a bidirectional road network like the attached image.

I was trying to submit the following to CSS Styling extension (GeoServer 2.9.1)

The CSS was accepted without validation errors but the generated SLD turned out to be the following

You will see that the interpretor just put the stroke-offset to PerpendicularOffset without any translation, not even wrapped in ogc:Literal tag. Consequently, GeoSever throws an error when this SLD is applied.

The correct SLD should be like this. This has worked without errors as GeoServer allows expressions for PerpendicularOffsett.

I think it is worth to enable users to use expressions in stroke-offset.
Please note that I have also tried this in the refined new style editor (the default one, not the extension one) but things were the same.

Many thanks,