A linking element's xlink:actuate attribute has 
four predefined
values:
 onRequest
 onLoad
 other
 none
<IMAGE 
  xmlns:xlink="http://www.w3.org/1999/xlink" 
       xlink:type="simple" xlink:href="logo.gif"
       xlink:actuate="onLoad"/>
Like all attributes in valid documents, the 
actuate attribute must be declared in the DTD 
in a <!ATTLIST> declaration for the link 
elements in which it appears. For example:
<!ELEMENT IMAGE EMPTY>
<!ATTLIST IMAGE 
 xmlns:xlink CDATA #FIXED "http://www.w3.org/1999/xlink"
  xlink:type CDATA #FIXED "simple"
  xlink:href CDATA #REQUIRED
  xlink:show    (new | replace | embed) "embed"
  xlink:actuate (onRequest | onLoad)    "onLoad"
>