A short history
The ICO format comes from Microsoft Windows, where it has stored application and system icons since the earliest versions. It found a second life on the web as favicon.ico — the little icon in a browser tab — a convention so entrenched that browsers still look for that exact filename automatically.
How it works
An ICO file is a container: it holds several images at different resolutions (commonly 16×16, 32×32 and 48×48) and bit depths, each stored internally as either BMP or PNG data, with transparency support. The operating system or browser then picks the size it needs for a given context.
When & how to use it
Use ICO for exactly one thing: icons — above all the classic favicon.ico. Bundle 16, 32 and 48 pixel versions so tabs, bookmarks and shortcuts all look crisp. It’s not a general-purpose image format; for anything larger or photographic, use PNG, WebP or SVG.
- A Windows format for application and system icons.
- A container that bundles multiple sizes (e.g. 16/32/48px) in one file.
- Each embedded image is stored as BMP or PNG data, with transparency.
- Still the automatic fallback browsers look for as “favicon.ico”.