[dspillett]

From the article:

> On the web it’s mostly used on the web for favicons though it can be used for normal images.

Basically the .ico format often used for favicons comes from Windows icons (the first favicon implementation originated in Internet Explorer and MS supported .ico so people could reuse existing application icon files, though IIRC .gif files and other formats supported in web pages could be used from the start too) which are a variant of the .bmp format which has a few extra features like containing different images for different sizes (so a single file can contain separately optimised 16x16 and 32x32 bitmaps).

I assume .ico and .bmp files are fairly rare now, presumably completely unheard of for new sites/apps, but you can't drop support because there are enough of them out there on legacy sites that it would break things people care about.

[gdwatson]

IE only supported .ico favicons for a long time.

[dspillett]

I've just looked it up (assuming https://en.wikipedia.org/wiki/Favicon#File_format_support is an accurate reference here) and was surprised that it seems IE11 was the first to support other formats.

I have kept our icons in .ico format but I thought that was to support proper legacy IE (5 and below). Where I have used other formats I presumably didn't notice the icon not working in IE8/9/10 because I generally only care about Firefox & Chrome and worry about IE10/11 breaking when someone reports a problem [if someone reports a problem in IE prior to 10 outside of my day job (where I have to support IE8 for some rather backward clients) I consider that their problem because of their browser choice, and even 10 is rapidly dropping off my care radar].