Your typical eink device (a kindle) is 6 inches diagonal, but there are also 6.8, 7.8, and 9.7 in somewhat common use, and more uncommon. Three sizes of iPads, dozens of sizes of android tablets, phones and laptops, etc.
You can't make a single page size display well on all of this -- at 8.5 x 11, a page will be too small to read when shrunk down to fit a 6in screen. Bake it at 6in and the pages are ridiculous on larger screens, but still too large for phones. You need a format that shapes the number and size of pages to fit the display -- enter ePub, which is based on HTML.
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