getElementsByTagName, CreateFakeImage

Hello there!

I'm working on a CMS, and we have tags like this in it:

<r:link page="3">Page three</r:link>

These tags doesn't show up in the editor, so I figured I might do something like what FCKeditor does with anchors and hidden input-fields.
This is my plugin code:

var FCKInternalTagProcessor = FCKDocumentProcessor.AppendNew() ;
FCKInternalTagProcessor.ProcessDocument = function( document )
{
  var aLINKs = document.getElementsByTagName('R:LINK') ;

  alert(aLINKs.length);

  var eLINK ;
  var i = aLINKs.length - 1 ;
  while ( i >= 0 && ( eLINK = aLINKs[i--] ) )
  {
    var oFakeImage = FCKDocumentProcessor_CreateFakeImage( 'FCK__InternalLink', eLINK.cloneNode(true) ) ;

    eLINK.parentNode.insertBefore( oFakeImage, eLINK ) ;
    eLINK.parentNode.removeChild( eLINK ) ;
  }
}

Most of the code is copied straight from the FCKeditor source, I only changed some variable names.
Anyways.
alert(aLINKs.length); always says there are 0 R:LINK-tags, even though there are more than 0. If I change all tags to R instead (<r page="3"></r>), and change the line in my plugin to document.getElementsByTagName('R');, it finds the tags, and the rest works too.
This got me thinking that getElementsByTagName can't find <r:whatever>-tags, so I tried it (http://nekasrof.ath.cx/~andreas/foobar.html), and it works!

Why does it work there, and not in FCKeditor?

Any ideas?

Thanks in advance