ensure external stylesheets are text contents

Former-commit-id: 390b334c1d8e895f00f53a3040599a8b0e54efa6

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

@@ -1593,7 +1593,8 @@ this.singlefile.lib.core = this.singlefile.lib.core || (() => {
 					maxResourceSize: options.maxResourceSize,
 					maxResourceSizeEnabled: options.maxResourceSizeEnabled,
 					charset: options.charset,
-					frameId: options.windowId
+					frameId: options.windowId,
+					validateTextContentType: true
 				});
 				resourceURL = content.resourceURL;
 				content.data = getUpdatedResourceContent(content.resourceURL, content, options);