I did not try the text shadow and did not have a retina screen, but for box-shadow MDN says that for the distribution radius:
"Positive values will cause the shadow to expand and grow larger, negative values will reduce the shadow. If not specified, it will be 0 (the shadow will be the same size as the element)"
This means that you can use:
box-shadow: 0 1px 1px -1px rgba(0, 0, 0, .5);
eg.
As I said, I have not tried the text shadow, but I suspect that it should work fine.
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