Fun with Favicons

Last week, Ray Sun wrote a short post about the origins of the Favicon, the little “site icon” you see on the tab when you visit a website in any major desktop browser. The cool thing about using the .ICO format as the basis of Favicons is that it is a container format that can contain multiple images of different size and color depth. Supporting multiple sizes is important because Favicons are now shown in many different places—the browser tab title, favorites list, new tab page, desktop shortcuts, Start Page shortcuts, etc.

Over on GitHub, Audrey Roy is putting together a “Favicon Cheat Sheet” which attempts to document the feature across browser versions; as you can see from her work, the feature has grown surprisingly complicated over the years. The guide is great (and improving) and I encourage you to check it out.

PNG Format

On Twitter, Audrey asked whether IE supports loading PNG files from Favicons; that’s desirable because unlike the default Bitmap format, PNG images are compressed.1

The answer is yes, IE on Vista+ supports RGB/a formatted PNGs inside ICO files. Support for PNG files was added to the system in Windows Vista and thus IE and the shell will use PNGs stored within the Favicon.

I put together a few simple test pages for Favicons. As you can see from this one, a favicon with both a PNG and a Bitmap will render the bitmap in IE on Windows XP and the PNG in IE on modern versions of Windows. Firefox and other browsers show PNG on all operating systems.


If you’d like to quickly see what image formats are included within a Favicon, you can select it in Fiddler and activate the ImageView Inspector. The formats within the icon file are listed in the sidebar:


Unfortunately, while my favorite icon-creation program supports editing PNG images within icons, it only offers to create icons using PNG when the image is 256x256. Fortunately, the icon format is quite simple (I wrote my own little icon compiler in C#) and you can use ImageMagick to quickly merge PNGs into an .ico file. (Update: An alert reader noted that when you use ImageMagick, it's embedding BMPs instead of PNGs within the .ICO for all formats except 256x256).

Favicon Caching

Favicons are shown in many different contexts, and hitting the network to download them on each use would be a performance nightmare. It would also lead to a confusing user-experience if the user showed a list of sites while offline, since all of the icons would be missing. To combat these problems, the system caches Favicons in a number of places.

First, Favicons are stored in the WinINET cache, subject to the same caching rules as other resources. (Tip: If you clear your browser cache but it seems like it’s holding on to an old favicon, be sure to uncheck the Preserve Favorites website data checkbox at the top of the Delete Browsing History dialog.)

Next, when an Internet Shortcut (“Favorite”) .url file is created on a NTFS volume (the default in all modern versions of Windows), an NTFS Alternate Data Stream is used to cache the icon alongside the shortcut itself. You can use the Streams tool to see this feature in use in your Favorites folder:


If you’d like to see the actual icon itself, try invoking your icon editor of choice like so:

C:\Windows\SysWOW64\mspaint.exe "C:\users\lawrence\favorites\Development Wiki.url:favicon:$DATA"

If you do use the Windows Paint program for this task, keep in mind that it only shows one of the images within the .ICO file; you can’t edit all of them.

By default, IE will pick up the favicon.ico file from the root of your server. If the user visits, IE will attempt to find the favicon at

However, individual pages can also specify the location of the favicon to use; a LINK element is used for this purpose. Prior to IE9, the element had to have the format

    <link rel="SHORTCUT ICON" href=""/>

This (arguably) reflects a mistake in the original design (since the rel attribute is meant to be a space-delimited list of tokens) and thus IE9 introduced support for specifying a relation of simply ICON, if you also specify the type as follows:

    <link rel ="ICON" type="image/x-icon" href=""/>

The Internet Explorer 11 preview allows omission of the type attribute, but if you want your icon to be used in IE9 and IE10 you should include it anyway.

Better in IE11

IE11 shows some significant improvements in Favicon support.

Of course, just because you can point at a PNG or GIF doesn't mean you should-- for both compatibility and experience reasons (remember .ico can hold multiple image sizes to best suit the display context) you should continue to use a .ico as your favicon.

Unfortunately, IE11 does not support specifying a Favicon using a DATA URI; while embedding the favicon in this way would have a number of downsides (including impairing performance in most cases) support for embedded/scriptable favicons could be useful in a few web application scenarios.


-Eric Lawrence

1 Note that you may, and usually should, use HTTP compression on traditional bitmap-containing favicons to avoid wasting gobs of bandwidth. Fun fact: For a time, the MSN favicon used more bytes than the entire Google homepage, including its logo JPEG!