Website favicons of MIME type image/png

By convention, the name of a website favicon image file must be favicon.ico. This name strongly implies that the MIME type of this file should be image/vnd.microsoft.icon. Indeed, IE does make this assumption: IE will only render website favicons in its own icon format.

Fortunately, neither Opera nor Firefox assumes that a favicon.ico file contains an image in Microsoft’s icon format. Firefox examines the content of an image file to discover the actual format of an image; this applies as well to favicon images. If Firefox recognizes the format as one it can render, Firefox cheerfully obliges. Opera’s behavior appears similar. Both Firefox and Opera will correctly render a website favicon prepared as a PNG image but saved as a favicon.ico file.

Note that Firefox will load website favicons and webpage favicons that are PNG images with 32-bit transparency. However, Firefox will save a favicon as a bookmark favicon only if the image size is less than 16K. This means that not all PNG website favicons or webpage favicons can be saved as bookmark favicons. For example, Macromedia Fireworks 8 will save a PNG file as either a Fireworks PNG or a Flattened PNG. Both image files are created with the extension .png. For a 16x16 pixel image, the size of the Fireworks PNG file may be 30K while the size of the Flattened PNG may be only 1K. Firefox will display webpage and website favicons provided in either PNG format. However, Firefox will not save a Fireworks PNG image as a bookmark favicon; instead, Firefox will substitute its default favicon for the intended bookmark favicon.

Although Opera and Firefox will accept additional website favicon image formats, they do not support other names for a website favicon file. They adhere to the favicon.ico convention. They will not request favicon files with different extensions (e.g., favicon.png).