CssBoxCreateBox 方法

Create new css box for the given parent with the given html tag.

Create new css box for the given parent with the given optional html tag and insert it either at the end or before the given optional box.
If no html tag is given the box will be anonymous.
If no before box is given the new box will be added at the end of parent boxes collection.
If before box doesn't exists in parent box exception is thrown.