If I contact favicon in my head from another domain, will it work fine? I want to use the same icon as an existing site, and my new site may turn out to be a completely different domain or subdomain of an existing one. Many thanks
Put this in the header:
<link rel="shortcut icon" href="your url here">
This will be fine, but divestment (bandwidth theft from another server) is not cool. Send it to your server or ask the webmaster for permission.
Yes, you can do this, favicon is treated like any other HTTP resources on the page, so it can be external.