HTMLZ is a compressed HTML file format that stores an HTML document along with its resources (like images and CSS). It’s often used to package websites for offline viewing or for sharing complete web pages. HTMLZ files can be opened in web browsers, which extract and render the content as a normal webpage.
AZW3 is a proprietary ebook format developed by Amazon for Kindle devices. It supports more complex content than older formats like AZW, including HTML5 and CSS3 elements, embedded fonts, and multimedia. AZW3 files provide better compatibility and richer features for modern Kindle devices and apps.