HTML is used in all browsers, while Flash needs a plugin to work. The older desktop version is built on Flash and therefore requires it to be accessible in the browser.
The Flash plugin is preinstalled in the majority of modern browsers. However, most iOS and Android devices do not support Flash. HTML is the fallback data associated with users accessing the web application from an iOS or Android device.
Just like you can create a segment for iOS and Android data, you can create a segment for HTML to find all HTML data.