[jdashg]

All-zeros isn't even really a valid ycbcr value, since even "full-range" "pc" is 1-255 for cb and cr! (Only full-range y has a range of [0,255])

[zamadatix]

I think 255 is reserved as well.

[jdashg]

Not in full-range. Narrow-range "tv" is [16,235] for Y and [16,240] (128 +/- 112) for Cb and Cr. Full-range is [0,255] and [1,255] (128 +/- 127) respectively.

The spec isn't that bad to read, though it's not loading for some reason right this moment: http://www.itu.int/rec/R-REC-BT.709-6-201506-I/en