extending attributes with an XHTML DTD

I am writing an XHTML extension, to add some attributes to all xhtml
objects, so that for any object, I can have "<a href="http://aol.com"
random="This is a note that is useful for me">

I tried the following dtd, and I'm not sure why it won't work: it
accepts the html tags in the file, but not any of the extended tags.
I'm not sure why. Any help would be wonderful!

____________________base-aaa.dtd_______________________

<!ENTITY % aaa-attrs
"required  (true | false)  #IMPLIED
readonly        (true | false)  #IMPLIED
random          %Text;          #IMPLIED"


<!ENTITY % xhtml11.dtd
        PUBLIC "-//W3C//DTD XHTML 1.1//EN"
                "http://www.w3.org/TR/xhtml11/D TD/xhtml11.dtd" >
%xhtml11.dtd;

<!-- Then add aaa-attrs to the common attributes -->

<!ENTITY % Common.attrib
    "%Core.attrib;
     %I18n.attrib;
     %Events.attrib;
     %aaa-attrs.attrib;" 
>