Erm, the alt attribute is a description, not a URI.

<!ELEMENT IMG - O EMPTY                -- Embedded image -->
<!ATTLIST IMG
  %attrs;                              -- %coreattrs, %i18n, %events -
+-
  src         %URI;          #REQUIRED -- URI of image to embed --
  alt         %Text;         #REQUIRED -- short description --
  longdesc    %URI;          #IMPLIED  -- link to long description
                                          (complements alt) --
  name        CDATA          #IMPLIED  -- name of image for scripting 
+--
  height      %Length;       #IMPLIED  -- override height --
  width       %Length;       #IMPLIED  -- override width --
  usemap      %URI;          #IMPLIED  -- use client-side image map --
  ismap       (ismap)        #IMPLIED  -- use server-side image map --
  >
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There's no reason to expect it's something that can be "followed".