I'm concerned about using block-level <div> tags for text formatting.

This changes the semantics, which effect things like text search in page. For example, in the demo page, do a text search on "is constrained". You will notice how it finds the text in both the Default Text and Balanced Text sections even though it wraps lines in the Default Text case. Now search on "the text" and you'll see that it is not found in the Balanced Text section because of the way the lines are formatted with <div> tags.

Also, the page may have margin/border/padding on <div> tags that would really mess this up.

Can this be implemented using inline <span> tags?