diff --git a/library/HTMLPurifier/URIFilter.php b/library/HTMLPurifier/URIFilter.php
index c116f93d..5d54c158 100644
--- a/library/HTMLPurifier/URIFilter.php
+++ b/library/HTMLPurifier/URIFilter.php
@@ -6,6 +6,18 @@
  * These filters can perform custom actions on a URI filter object,
  * including transformation or blacklisting.
  *
+ * The following contexts may be available while URIFilters are being
+ * processed:
+ *
+ *      - EmbeddedURI: true if URI is an embedded resource that will
+ *        be loaded automatically on page load
+ *      - CurrentToken: a reference to the token that is currently
+ *        being processed
+ *      - CurrentAttr: the name of the attribute that is currently being
+ *        processed
+ *      - CurrentCSSProperty: the name of the CSS property that is
+ *        currently being processed (if applicable)
+ *
  * @warning This filter is called before scheme object validation occurs.
  *          Make sure, if you require a specific scheme object, you
  *          you check that it exists. This allows filters to convert