It’s considerably more complicated than that. Unreal is largely very good when it comes to allowing you to build a game for multiple platforms; the export process for any given platform basically involves setting up a config file and pressing a button - how you implement required basic platform specific features within your game is of course your responsibility (for example input handling and user interfaces). For consoles, you will need additional agreements with those companies that allow you access to their specific closed branches of the engine, and you’ll need to maintain your custom version of the engine with respect to theirs.
For online play in any given platform, you will generally use that platform’s infrastructure, as provided in the relevant SDK; Steam has Steamworks, XBox has Live and so forth. The associated closed branches of those engines will give you the ability to integrate said platform infrastructure with your game - doing so is generally a requirement as part of your platform agreement with regard to consoles. Matchmaking, party systems, voice chat, all those features are part of the platform online services infrastructure.
The moment you want to go cross-platform - ignoring associated legal nightmares due to platform agreements - you’re going to have to scrap all the aforementioned useful features like matchmaking etc, and implement your own infrastructure to run your game, all whilst still conforming to the standards required by each platform and retaining the required features. Unsurprisingly, this isn’t a small amount of work - but worse, you need to maintain your own services infrastructure, which is expensive as you’ll need dedicated staff and hardware for the job.
In any browser and on a platform that meets the requirements, but for numerous reasons using UE4 to produce HTML5 products isn’t really worth the hassle.
Thank you for that answer, ambershee! My native language isn’t English so bare with me if I’m just asking you to repeat something. Say I do a game for PC, Android and iOS, would I only be required to setting up the config file for it to work? At least the basics, the game could be a single player Solitare with no score board.
Also, what would the HTML5 part be for? Is it mostly there to future prof Unreal Engine or does it have any preferred areas of use?