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Nope. I rechecked the PNG specification, and it explicitly states that the high bits must be zero:

"If the image bit depth is less than 16, the least significant bits are used and the others are 0." -- Year 2003, https://www.w3.org/TR/2003/REC-PNG-20031110/#11bKGD

"Gray: 2 bytes, range 0 .. (2^bitdepth)-1" -- Year 1996, version 1.0, http://www.libpng.org/pub/png/spec/1.0/PNG-Chunks.html#C.bKG...

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It makes sense to treat it as an error but the ISO standard is too vague on this and the libpng developers never "fixed" this[1], it issues a warning but otherwise just masks off the high bits. Firefox has a workaround[2][3], Chrome doesn't.

FWIW I do the same in my library[4], handling it as an error is problematic because it would mean the chunk gets discarded and then you can't even retrieve the values.

[1] https://sourceforge.net/p/png-mng/mailman/png-mng-implement/...

[2] https://bugzilla.mozilla.org/show_bug.cgi?id=428045

[3] https://searchfox.org/mozilla-central/source/image/decoders/...

[4] https://libspng.org/

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