An explanation of... many things, perhaps?
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And if anyone is wondering why I care: one of the other reasons to use DocBook is that it makes it straightforward to generate two separate versions of the same document with different layouts. For example, one intended for reading online, and one intended for printing; despite assertions about ‘one size fits all’, monitors and printed books have extremely different requirements for ‘best’ or even ‘good’ usage, and tend to only meet around a ‘barely usable common denominator’. It’s more of a ‘one size fits all… if that one size is a circus tent bound with rubber bands that cut off the circulation and then strategically stapled to the wearer in sensitive places, such as the eyes’ situation.