You use the webapp from your target device, then when you select a book, it's converted on the server (I use mostly Kindle Comic Coverter and Calibre ebook-conv

You use the webapp from your target device, then when you select a book, it's converted on the server (I use mostly Kindle Comic Coverter and Calibre ebook-convert), then served as a regular html download and opened in a native reader. This works particularly well from Kindle, so I never have to connect it to a computer, and I don't have any email size limit which is quite easy to go over with comics