Are you running some sort of page-speed optimization service or compression service in front of your site? Or perhaps some code in your theme or a plugin that attempts to detect and behave differently for Windows computers?
I ask this because when viewing your site on Chrome and Firefox on a Mac, I see this. However, when loading in Chrome or Internet Explorer on Windows, I get this.
I can see what's causing it in the code — your site is outputting incomplete code to Windows browsers (screenshot of code, with commentary).
You might try deleting and reinstalling the theme, especially if customizations have been made to it. (Make a backup first if you have edited it!)
If that doesn't trigger a bell, I'd have to look at your site's setup in the admin. Would you be able to send a temporary admin login at email@example.com? Enabling Styles Debug Mode in Settings > Styles might also give me some additional hints.