style:writing-mode refers to XSL 1.0, 7.27.7 to define the values of this attribute, except for "page."

But, we include tb-lr, which I assume means top to bottom, left to right, which should have the same result as lr-tb, but it isn't defined in XSL 1.0. (nor do we define it)

Two steps:

Confirm lack of reliance in implementations

Deprecate with note on next version it will be removed.