Earlier quoted context omitted.
> The implementation will look vastly different if you define 'where' as a printer, desktop, tablet, mobile phone, wearable device, etc. Not since more than a decade ago, when we started using the same rendering infrastructure (e.g Display Postscript, Quartz, Cairo, what Windows uses etc) for both print and display. In fact nowadays, with hidpi (retina) displays, you even get similar resolution between the average pr…
I have a couple HTML documents open on my MacBook, iPad, and iPhone. All three look different. I opened a couple of eBooks on Kindle and iBooks on my MacBook, iPad, and iPhone. All three look different. All three use Display Postscript, CoreGraphics and Quartz. All three look different. WYSIWG depends on where. I don't own a printer, but I bet if I printed from all three devices all three documents would look differe…
Your ebooks are also not WYSIWYG. They're also specifically designed to display differently on different devices, and according to user preferences for things like font, text size, brightness, background, line spacing, etc..
WYSIWYG is exactly the opposite of these formats, and does not depend on the device. Postscript and PDF are WYSIWYG.