If you zipped the html5_output folder and sent that to them, it's likely that they are trying to open the index.html page without first extracting the zip file.
What's happening is Windows lets you preview the zip file without opening it. The preview cannot access the rest of the files that it needs (such as the CSS, the JavaScript, and the images) to render the content in the browser until they're unzipped.
You can confirm that this is the issue by looking at the address bar in the browser. You'll see ".zip" in the file path.
The easiest way to unzip on Windows is to right-click and choose Extract all... then follow the steps to zip the output to a folder.
