Concept:
Block-container is a block-level formatting object that wraps blocks into a bigger block area with additional attributes to control its size, writing-mode, reference-orientation, etc. Its content is limited to a sequence of blocks only.
A "block-container" formatting object has the following main attributes:
- width="..." and height="..." - Optional attributes to specify the width and height of output block area. Default values are automatically calculated.
- writing-mode="..." - Similar to region-body[@writing-mode].
- reference-orientation="..." - Similar to region-body[@reference-orientation].
- Other attributes are similar to "block" elements.



