Anthing that is not text. This could be an image or an inline-block element.
For now, we assume that these beasts are not breakable at the end of the
line (outer linebreaks).
author: Thomas Morgner
The extra-space width for an element. Some elements can expand to fill some
more space (justified text is a good example, adding some space between the
letters of each word to reduce the inner-word spacing).