2.1.7 [W3C-PNG] Section 126.96.36.199, sBIT Significant bits
The specification states:
188.8.131.52 sBIT Significant bits Each depth specified in sBIT shall be greater than zero and less than or equal to the sample depth (which is 8 for indexed-colour images, and the bit depth given in IHDR for other colour types). Note that sBIT does not provide a sample depth for the alpha channel that is implied by a tRNS chunk; in that case, all of the sample bits of the alpha channel are to be treated as significant. If the sBIT chunk is not present, then all of the sample bits of all channels are to be treated as significant.
All Document Modes (All Versions)
The sBIT chunk is not supported.