Revert "remove htmlpurifier"

This reverts commit c21a462d52.

lib/htmlpurifier/library/HTMLPurifier/AttrDef/CSS/AlphaValue.php

@@ -0,0 +1,21 @@
+<?php
+
+class HTMLPurifier_AttrDef_CSS_AlphaValue extends HTMLPurifier_AttrDef_CSS_Number
+{
+
+    public function __construct() {
+        parent::__construct(false); // opacity is non-negative, but we will clamp it
+    }
+
+    public function validate($number, $config, $context) {
+        $result = parent::validate($number, $config, $context);
+        if ($result === false) return $result;
+        $float = (float) $result;
+        if ($float < 0.0) $result = '0';
+        if ($float > 1.0) $result = '1';
+        return $result;
+    }
+
+}
+
+// vim: et sw=4 sts=4