> For BMPs, for example, determining whether the author intended 24-bit RGB or 32-bit RGBA sometimes requires decoding the full image and scanning to see whether any pixels' alpha bytes differ from the others, since "all 00" and "all FF" might both be "no alpha".

This causes a situation in which a page that renders the image in Chrome doesn't work in Firefox or other browsers that don't implement the same non-standard correcting algorithm. Worse, the user doesn't have a way to know from Chrome that the image is broken and it will likely continue to be broken forever.

Generalizing this approach, you end up having to test your site in every major browser to see if you didn't made a mistake that is only revealed in the browser which lacks that recovery mechanism.

> I also used to work on a production C compiler. Compilers can and do "guess what you meant" in various cases, notably for producing actual human-readable errors or proceeding past various warnings, but if I recall correctly even in more obscure non-error cases.

I don't think this is addressing my point. When you write a C program, you expect the compiler to either recognize the program from the C grammar, or reject it because it is not correct (hence the concept of "error"). Then run whatever guessing algorithm to report to you what may be wrong.

The programmer expectation is that the program must strictly conform to the C grammar, and errors are corrected. It is not silently producing a half-reconstructed program assuming what you meant to say.