Favicons can also be used to have a textless favorite site, saving space.
In March 1999, Microsoft released Internet Explorer 5, which supported favicons for the first time. Originally, the favicon was a file called
favicon.ico placed in the root directory of a website. It was used in Internet Explorer's favorites (bookmarks) and next to the URL in the address bar if the page was bookmarked. A side effect was that the number of visitors who had bookmarked the page could be estimated by the requests of the favicon. This side effect no longer works, as all modern browsers load the favicon file to display in their web address bar, regardless of whether the site is bookmarked.
The favicon was standardized by the World Wide Web Consortium (W3C) in the HTML 4.01 recommendation, released in December 1999, and later in the XHTML 1.0 recommendation, released in January 2000. The standard implementation uses a link element with a
rel attribute in the
<head> section of the document to specify the file format and file name and location. Unlike in the prior scheme, the file can be in any Web site directory and have any image file format.
In 2003, the
.ico format was registered by a third party with the Internet Assigned Numbers Authority (IANA) under the MIME type
image/vnd.microsoft.icon. However, when using the
.ico format to display as images (e.g. not as favicon), Internet Explorer cannot display files served with this standardized MIME type. A workaround for Internet Explorer is to associate
.ico with the non-standard
image/x-icon MIME type in Web servers.
RFC 5988 established an IANA link relation registry, and
rel="icon" was registered in 2010 based on the HTML5 specification. The popular
<link rel="shortcut icon" type="image/x-icon" href="favicon.ico"> theoretically identifies two relations, "
shortcut" and "
icon", but "
shortcut" is not registered and is redundant. In 2011 the HTML living standard specified that for historical reasons "
shortcut" is allowed immediately before "
icon"; however, "
shortcut" does not have a meaning in this context.
Internet Explorer 5–10 supports only the ICO file format. Netscape 7 and Internet Explorer versions 5 and 6 display the favicon only when the page is bookmarked, and not simply when the page is visited as in later browsers.
The following table illustrates major web browsers supporting different features. The version numbers indicate the starting version of a supported feature.