[css2] BIG edit. The primary changes involved reorganizing the contents to clarify the box model vs the flow model. Other changes involved clarifying four edges: margin, padding, border, and content. Lots of clarifications of language (element vs. box), also some minor fixes, like <strong> used with all <dt>, fixed buggy HTML in examples, etc.

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extra : convert_revision : svn%3A73dc7c4b-06e6-40f3-b4f7-9ed1dbc14bfc/trunk%40580

Author: ianbjacobs