HTML export: Add validation link to page

An exported document can now have a link for XHTML validation
included.

Patch by Sebastian Rose

lisp/ChangeLog

@@ -3,6 +3,8 @@
 	* org-exp.el (org-export-as-html): Add a "content" div around the
 	entire content of the body tag.
 	(org-export-html-get-bibliography): New function.
+	(org-export-html-validation-link): New variable.
+	(org-export-as-html): Add validation link to exported page.
 
 	* org.el (org-match-sparse-tree): Renamed from
 	`org-tags-sparse-tree'.

lisp/org-exp.el

@@ -746,6 +746,11 @@ This is customizable so that alignment options can be specified."
   :group 'org-export-tables
   :type '(cons (string :tag "Opening tag") (string :tag "Closing tag")))
 
+(defcustom org-export-html-validation-link nil
+  "Non-nil means, add validationlink to postamble of HTML exported files."
+  :group 'org-export-html
+  :type 'boolean)
+
 (defcustom org-export-html-with-timestamp nil
   "If non-nil, write `org-export-html-html-helper-timestamp'
 into the exported HTML text.  Otherwise, the buffer will just be saved
@@ -4072,6 +4077,8 @@ lang=\"%s\" xml:lang=\"%s\">
 	  (when org-export-creator-info
 	    (insert (format "<p class=\"creator\">HTML generated by org-mode %s in emacs %s</p>\n"
 			    org-version emacs-major-version)))
+	  (when org-export-html-validation-link
+	    (insert "<p class=\"xhtml-validation\"><a href=\"http://validator.w3.org/check?uri=referer\" target=\"_blank\">Validate XHTML 1.0</a></p>"))
 	  (insert "</div>"))
 
 	(if org-export-html-with-timestamp