Revert "remove htmlpurifier"

This reverts commit c21a462d52.

lib/htmlpurifier/library/HTMLPurifier/AttrTransform/Background.php

@@ -0,0 +1,23 @@
+<?php
+
+/**
+ * Pre-transform that changes proprietary background attribute to CSS.
+ */
+class HTMLPurifier_AttrTransform_Background extends HTMLPurifier_AttrTransform {
+
+    public function transform($attr, $config, $context) {
+
+        if (!isset($attr['background'])) return $attr;
+
+        $background = $this->confiscateAttr($attr, 'background');
+        // some validation should happen here
+
+        $this->prependCSS($attr, "background-image:url($background);");
+
+        return $attr;
+
+    }
+
+}
+
+// vim: et sw=4 sts=4