Does a favicon have to be 32x32 or 16x16?
I'd like to use a single image as both a regular favicon and iPhone/iPad friendly favicon.
Is this possible? Would an iPad-friendly 72x72 PNG scale if linked to as a regular browser favicon? Or do I have to use a separate 16x16 or 32x32 image?
Answer
Update for 2020: Sticking to the original question of 16x16 versus 32x32 icons: the current recommendation should be to provide a 32x32 icon, skipping 16x16 entirely. All current browsers and devices support 32x32 icons. The icon will routinely be upscaled to as much as 192x192 depending on the environment (assuming there are no larger sizes available or the system didn't recognize them). Upscaling from ultra low resolution has a noticeable effect so better stick to 32x32 as the smallest baseline.
For IE, Microsoft recommends 16x16, 32x32 and 48x48 packed in the favicon.ico file.
For iOS, Apple recommends specific file names and resolutions, at most 180x180 for latest devices running iOS 8.
Android Chrome primarily uses a manifest and also relies on the Apple touch icon.
IE 10 on Windows 8.0 requires PNG pictures and a background color and IE 11 on Windows 8.1 and 10 accepts several PNG pictures declared in a dedicated XML file called browserconfig.xml.
Safari for Mac OS X El Capitan introduces an SVG icon for pinned tabs.
Some other platforms look for PNG files with various resolutions, like the 96x96 picture for Google TV or the 228x228 picture for Opera Coast.
Look at this favicon pictures list for a complete reference.
TLDR: This favicon generator can generate all these files at once. The generator can also be implemented as a WordPress plugin. Full disclosure: I am the author of this site.