All websites should be following the favicon rule.
A Favicon is a small image file included on nearly all professional developed sites. When a browser hits your website and a user bookmarks that site then the favicon.ico graphic will be displayed in the browser’s URL/address line upon subsequent visits to that site.
Let's see how it's done for SharePoint:
<head runat="server">
<meta name="GENERATOR" content="Microsoft SharePoint">
<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
<!--Placeholder for additional overrides-->
<asp:ContentPlaceHolder ID="PlaceHolderAdditionalPageHead" runat="server" />
<link rel="shortcut icon" href="~/Style Library/Images/SSW/Rules/ssw.ico" type="image/x-icon" />
</head>
Figure: One line of HTML lets you add your company's icon to your web page
