"favicons" are small icons that most browsers display to help users identify websites. Note that all browsers support bitmap images, but only certain modern browsers support SVG format icons.
When TiddlyWiki starts up in the browser it looks for a tiddler called $:/favicon.ico and dynamically uses it as the favicon for the page. If you modify the image then the favicon changes instantly to reflect it.
New in: 5.1.23 To set an external image as the favicon, assign the URL to the _canonical_uri field of the $:/favicon.ico tiddler.