>Gamma, Color Space Information, Physical Dimensions, and Background Color

Gamma is assumed to be sRGB gamma by default. Default color space is likewise sRGB. The Background Color is supposed to be used as the background color, against which the image is alpha-composited by viewers. Most modern viewers ignore it, because the whole point of the alpha channel is that it an be shown over any color.

>Meanwhile the Trailer IEND has an uppercase letter, indicating the zero-length outro byte is somehow more essential than all the color info

The IEND chunk was made mandatory, to make the format self-delimiting. i.e. a PNG file truncated at a chunk boundary would look just like a complete file. (although, one could argue that you could find out it's truncated when the uncompressed data comes back smaller than widthheightbytes_per_pixel)