Please help me keep the forums organized by asking only one question/topic per thread.
I don't think links should open in new windows, target is not a valid attribute in xhtml nor in html 5. See the article Links Should Not Open in a New Window for information why and also for a way to do it if you really want to open in new windows anyway.
I will make it possible to use a CSS class on the Gravatar link like this:
if (CssClass.Length > 0)
then you could set a class in theme.skin like this:
<portal:Gravatar runat="server CssClass="external" />
Hope that helps,