Only C-grade for Maemo 5 browser?

Only C-grade for Maemo 5 browser?

I checked the browser support chart and noticed that the native browser ( MicroB) for Maemo 5 only has a C-grade. As it Mozilla-based and uses the Gecko layout engine (version 1.9?) it has full CSS and Javascript support and should have a higher grade, or is the reason for the C-grade its low market share?
I have also heard rumors that Nokia is rewriting its native Symbian browser to be based on MicroB in the future.