Try to add block macro for iframe.

build.gradle

@@ -35,15 +35,15 @@ asciidoctor {
 		'orgname':'jMonkeyEngine'
 	options header_footer: true,
 		template_dirs: [file('src/templates/slim').absolutePath]
-/*
+
 	extensions {
 		block_macro(name: 'iframe') {
 			parent, target, attributes ->
-			String content = """<iframe src="${target}"></iframe>""";
+			String content = +++<iframe src="${target}"></iframe>+++;
 			createBlock(parent, "pass", [content], attributes, config);
 		}
 	}
-*/
+
 }
 
 asciidoctor.doLast {