Probably with some kind of data_source classes to which will have same interface so that they could be easily rendered. For now I think that this should be implemented as separate lib.

Solution for adding display attributes. I think that best will be in form:

$attribute->required->style = "color: red;";
$attribute->normal->style = "color: blue;";
$attribute->error->class = "error";

This solution is implemented in kodform4.

Should have support for xforms but as separate classes or member function ( $form→displayX() ).