FXL ePubs, Reflowable ePubs and InDesign's Publish Online facility are all based on HTML5. Each has advantages (features) and disadvantages. For example, as James explained, Publish Online can only be hosted on Adobe servers. There's a new feature in InDesign 2025, where you can export an InDesign document as HTM5, but it's in a beta version and should only be used for experimentation at this stage. The other option you might consider is in5, an InDesign plug-in (at extra cost). https://ajarproductions.com/
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