The current spec for text-shadow states:

unlike box-shadow, the spread distance is strictly interpreted as outset distance from any point of the glyph outline, and therefore, similar to the blur radius, creates rounded, rather than sharp, corners.

@smfr writes:

I don't think we can say that spread results in rounded corners. As an implementor I'd implement spread by increasing the stroke width, so the corners will still respect the miter limit that's used to render the glyph outlines, which could result in long spikes. This probably needs to be informed by some implementation experience.

Do we want to spec anything particular here, or leave it up to the UA for now?