If you use data in degree format, as opposed to the faster and more usual method of using pre-projected data, you can decide the projection at the time the application runs, and specify it in the style sheet. You can use different projections for different scales if you like.
To do this, use the <projection> element at the top level of the style sheet (the same level as the <layer> elements). Its attributes are
If you prefer, and this may make the style sheet more readable, you can enclose the <projection> element in a <scale> element instead of using minScale and maxScale.