As of Unreal Engine 4.24, support for As of Unreal Engine 4.24, support for the HTML5 platform has been migrated out of the engine to a public Platform Extension that can be maintained and improved by community members. Documentation for the HTML5 Platform Extension is hosted alongside the GitHub repository .
Here you can read more about trying to get latest Unreal Engine working with HTML5 platform extensions: UE4 HTML5 Platform Extension