I created a little HTML file that displays Browser User Agent and tried to add it to Lectora via Web Window function. This is what I got (see pic). Apparently, the window for the preview is based on inclusion of some ancient browser (that probably comes with whatever framework Lectora is built upon). The user agent string looks like a mix of IE7 and IE6.
So when you preview any HTML content (web-based or local) in that tiny preview window, it may break or throw warnings. Maybe it would make sense to use the same Awesomium plugin that the main Lectora window uses.