Requirement: allow MapML author to specify tile opacity with CSS

[prushforth]

This would avoid having to mess around with creating many separate MapML services where a single one which combined related layers would do. For example, hill shading/relief could be overlayed by transportation network, which is itself overlayed by a text layer. Each could have an appropriate opacity assigned by CSS rules.

[prushforth]

A major negative of not supporting this feature is that it pushes complexity on to the HTML author AND the end user, which is worse still. So it's pretty much a must-have.

[prushforth]

The z-order is of course already under the control of the MapML author, by virtue of the order in which the tile elements are placed in the document. So the requirement goes down to tile opacity, which should be specifiable via CSS rules.