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A nice thing about the initial ico-format is that it can contain multiple resolutions. Nowadays you will see multi-res using separate files but that took a long time to happen.

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Indeed, I think ICO is the only common-use format to support that (I guess JPEG "sort of" does support it but not in any usable multi-resolution way).

Mipmaps are common in video game graphics format though!

macOS's .icns files (typically used for app icons) can (and usually do) also store multiple resolutions in a single file.

> common-use format

Pretty much nothing outside the apple ecosystem supports .icns

I believe that with svg, you can have multiple breakpoints and style rules depending on canvas size.

You can set the shortcut icon meta tag to point to a fairly big (256x256) png file, and all modern browsers will re-scale it for use where-ever needed.

The only place where I've seen multiple sizes needed was for SPAs on mobile phones etc. and that's a whole different set of tags.

Sometimes you want more control over the low-res result than a re-scale will give.

Use SVG with CSS media queries.

At the scales we're taking about, bitmap image is both faster and smaller than equivalent SVG & CSS with media queries.

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