src/Pure/Thy/present.scala

 
       File.copy(Path.explode("~~/etc/isabelle.css"), session_prefix)
 
       for (font <- Path.split(Isabelle_System.getenv_strict("ISABELLE_FONTS")))
         File.copy(font, session_prefix)
+      for (font <- Path.split(Isabelle_System.getenv_strict("ISABELLE_FONTS_HTML")))
+        File.copy(font, session_prefix)
     }
   }
 }