Format compliance
ICO output is a real multi-resolution Windows icon, not a renamed PNG. PNG output is encoded with the standard PNG chunks and a correct CRC. JPEG output respects the chosen quality and includes a valid JFIF header. We test each format against the operating system's native renderers (Windows Explorer, macOS Finder, Linux file managers).
Visual fidelity
Resizing uses high-quality bicubic interpolation backed by the browser's canvas implementation. Compression presets are tuned against a corpus of real product photos, illustrations, and screenshots — not synthetic test images.
Bug reports
If you ever get an output that doesn't open in the expected program, send us the input file at hello@fetchfavicon.com. We treat output bugs as P0 and ship a fix in the next release.