This thread describes some difficulties in implementing spread on text-shadow. The gist is that spread on box-shadow can be implemented via changing rectangle sizes and adjusting corner radii, but spread on text-shadow would have to be implemented by shadowing the text rendered with an enlarged stroke, which could lead to unexpected shadowing of parts of glyphs in some fonts (I saw issues with Zapfino).

In addition, the stroke-enlarging implementation doesn't allow for negative spread, so I suggest we disallow negative spread on text-shadow.