The default favicon searched by the browser is:
/favicon.ico
It's equivalent to the below code:
<link rel="icon" href="/favicon.ico" />
The ico file format may be:
A icon is declared through the link element in a HTML page and every browser may have a different way to define it.
<link rel="shortcut icon" href="http://gerardnico.com/wiki/lib/tpl/bootie/images/favicon.ico" />
Chrome and Opera uses icon.png, which is scaled to the necessary size by the device. To prevent automatic scaling, you can also provide additional sizes by specifying the sizes attribute.
<!-- icon in the highest resolution we need it for -->
<link rel="icon" sizes="192x192" href="icon.png">
<!-- and this one also -->
<link rel="icon" type="image/png" href="/assets/favicons/favicon-32x32.png" sizes="32x32">
<link rel="icon" type="image/png" href="/assets/favicons/android-chrome-192x192.png" sizes="192x192">
<link rel="icon" type="image/png" href="/assets/favicons/favicon-96x96.png" sizes="96x96">
<link rel="icon" type="image/png" href="/assets/favicons/favicon-16x16.png" sizes="16x16">
Safari also use the link tag with the rel attribute: apple-touch-icon to indicate the home screen icon.
<link rel="apple-touch-icon" sizes="57x57" href="/assets/favicons/apple-touch-icon-57x57.png">
<link rel="apple-touch-icon" sizes="60x60" href="/assets/favicons/apple-touch-icon-60x60.png">
<link rel="apple-touch-icon" sizes="72x72" href="/assets/favicons/apple-touch-icon-72x72.png">
<link rel="apple-touch-icon" sizes="76x76" href="/assets/favicons/apple-touch-icon-76x76.png">
<link rel="apple-touch-icon" sizes="114x114" href="/assets/favicons/apple-touch-icon-114x114.png">
<link rel="apple-touch-icon" sizes="120x120" href="/assets/favicons/apple-touch-icon-120x120.png">
<link rel="apple-touch-icon" sizes="144x144" href="/assets/favicons/apple-touch-icon-144x144.png">
<link rel="apple-touch-icon" sizes="152x152" href="/assets/favicons/apple-touch-icon-152x152.png">
<link rel="apple-touch-icon" sizes="180x180" href="/assets/favicons/apple-touch-icon-180x180.png">
<link rel="mask-icon" href="/assets/favicons/safari-pinned-tab.svg" color="#5bbad5">
You can leave out the relevant meta tags if you don't want to support a specific size.
<meta name="msapplication-TileColor" content="#7d3194">
<meta name="msapplication-TileImage" content="/assets/favicons/mstile-144x144.png">
<!-- Or -->
<meta name="msapplication-square70x70logo" content="icon_smalltile.png">
<meta name="msapplication-square150x150logo" content="icon_mediumtile.png">
<meta name="msapplication-wide310x150logo" content="icon_widetile.png">
<meta name="msapplication-square310x310logo" content="icon_largetile.png">
It can also be declared on the web server.
It's location default to the root with the name favicon.ico
Example: https://maven.apache.org/favicon.ico