Making a note of this since I always forget…

Tracking and kerning in InDesign are both measured in 1/1000 em, which is relative to the current type size. To map these values to CSS ems you just multiply by .001.

Here are some examples with InDesign tracking in the comments:

/* -40 x .001 */
letter-spacing: -0.04em;

/* 5 x .001 */
letter-spacing: 0.005em;

/* 20 x .001 */
letter-spacing: 0.02em;