255 gives 0-255, which gives you a zero value. 256 is 1-256, you lose the option of setting 0.

If you have a ruler and it goes to 12 inches, you should normalize by the length L and not by 13, the number of points on the ruler.

I'll argue for the +0.5 solution. First, I don't like half-sized intervals at the edges, and second, a 255-based representation is typically a SDR (not HDR) image.

RGB values represent luminances against some adapted state, and a "zero" in a daylit scene is not "zero luminance" - it's just about 0.001x as bright as the brightest point - it's millions of photons, way more than zero. In a sense our eyes experience contrast on a sliding scale, and there is no absolute zero in the system. For example, broadcast systems historically used 16-235 as their luminance range for SDR. I think any argument that says "we must have zero" is going to have a bias, but I don't think zero is needed for most things.

Should always be 0-255 as that fits an unsigned byte.

Both of these assume a linear transfer function, which is rarely the case.

Advice for anyone on mobile: read in landscape mode if you want to be able to see the division by 256 version code example at the start.

The HTML/CSS is bad that lets it completely overflow the right edge of the page instead of wrapping.

I re-read this post three times in total confusion before I figured out the most important piece was off-screen entirely.