- implement few positioning properties: position, width, height, anchor; - implement font element; - implement fill element (used by font) and its children (linear, radial, ...); - font property is inherited from ancestor container (see https://www.pdfa.org/wp-content/uploads/2020/07/XFA-3_3.pdf#page=43) so let CSS handles that stuff; - in order to reduce the number of properties to set, only set non default properties and put the default in CSS; - set a background to some containers to be able to see them (will be removed in a future commit).