Don't think this is a high priority option for them. It is possible to switch on, but apparently it would impact on connected platforms if I recall from a last discussion about this some time back.
If they can get away with their own books being formatted by some other means, I don't think they'll invest more on Inkstone than required. The only way I see HTML being switched on is when Inkstone is the primary publishing application that all their paid books go through.