fixed sandbox attribute value

lib/single-file/single-file-core.js

@@ -477,7 +477,7 @@ this.SingleFileCore = (() => {
 						if (frameData.processor) {
 							const pageData = await frameData.processor.getPageData();
 							frameElement.setAttribute(frameElement.tagName == "OBJECT" ? "data" : "srcdoc", (frameElement.tagName == "OBJECT" ? "data:text/html," : "") + pageData.content);
-							frameElement.sandbox = true;
+							frameElement.setAttribute("sandbox", "");
 						}
 					}
 				} else {