Live Preview (July 2018 update)
I've suggested this before, but the idea was dismissed at the time. Since I can't re-open the thread, I just wanted to add another example I found, where the concept seems to be useful, in an attempt to convince :)
Statamic enables live preview through VueJS, showing you the site changing as you type, which you can see on their homepage — under the 'See it in action' heading.
'Live' preview now seems to be a common feature amongst lots of CMS', as well as excellent UX.
Statamic, Craft and WordPress (via Gutenberg) now have some variety of a 'live' editing experience where you can see the front-end of a website live updating as you interact with the admin.