diff -r d07300e8a14d -r ee4cf96a9406 src/Pure/General/http.scala
--- a/src/Pure/General/http.scala	Thu Jun 01 21:24:33 2017 +0200
+++ b/src/Pure/General/http.scala	Thu Jun 01 21:43:36 2017 +0200
@@ -137,7 +137,7 @@
 
   private lazy val html_fonts: SortedMap[String, Bytes] =
     SortedMap(
-      Isabelle_System.fonts(html = true).map(path => (path.base.implode -> Bytes.read(path))): _*)
+      Isabelle_System.fonts(html = true).map(path => (path.base_name -> Bytes.read(path))): _*)
 
   def fonts(root: String = "/fonts"): Handler =
     get(root, uri =>