In SharePoint master pages the
<SharePoint:CssLink> plays a pivotal role in not only referencing default CSS file, but in loading CSS files based on the SharePoint version. For instance, if you look at the
<SharePoint:CssLink> in the v4.master page, it appears in the
<head> section as the following
<SharePoint:CssLink runat="server" Version="4"/>
Version 4 indicates SharePoint 2010, and Version 3 indicates SharePoint 2007, included specifically for upgrade purposes. If Version is set to 3 then at runtime SharePoint loads the Version 3 CSS files from the Web front-end, including core.css, instead of the Version 4 CSS files.
In addition, the
<SharePoint:CssLink> also discovers any customized CSS ...