Suggestions

Umbraco version: 4.0.3 (and possible later?)

Due to better SEO and faster rendering of sites with slow conections/heavy script dep. *
- use another aproach for registring (1) javascript files and (2) startup scripts.

* More info see: http://developer.yahoo.com/performance/rules.html#js_bottom

1) As it is now core library method RegisterJavaScriptFile uses the header to put additional script include files; both backup and the .net standard way - wich is *flawed*  IMHO considering SEO and deffered loading)

source:

                System.Web.UI.HtmlControls.HtmlGenericControl include = new System.Web.UI.HtmlControls.HtmlGenericControl("link");
                include.ID = key;
                include.Attributes.Add("rel", "stylesheet");
                include.Attributes.Add("type", "text/css");
                include.Attributes.Add("href", url);

                if (p.Header != null)
                {
                    if (p.Header.FindControl(key) == null)
                    {
                        p.Header.Controls.Add(include);
                    }
                }
                else
                {
                    //This is a fallback in case there is no header
                    p.ClientScript.RegisterClientScriptBlock(p.GetType(), key, "<link rel='stylesheet' href='" + url + "' />");
                }

Some suggested alternatives:

* Use p.ClientScript.RegisterStartupScript instead, as this method place the script next to closing form

* Use http-module to render all script below visible content -

see http://weblogs.asp.net/omarzabir/archive/2008/04/06/fast-page-loading-by-moving-asp-net-ajax-scripts-after-visible-content.aspx for more info.

2) There is no register startupscript block to use from library inside lets say a macro. (one has to use Umbraco library  - RegisterClientScriptBlock that renders the script with .NET RegisterClientScriptBlock

Suggested solution:
* expose a new method RegisterStartupScript utilizing .NET's method: Page.ClientScript.RegisterStartupScript .