I like the idea, but I'm not certain how to make it useful beyond "casual reading about devtools".

The very first tip I see is "Customize the way objects look" which is ok... cool you can do that, is this where it should start? Also, it seems Safari only got a handful of tags and many of its unique (and very useful) features are missing completely from the list.

I mean, why not? It's not all for fun and giggles, it's actually a very useful feature, I use it a lot when working with data structures I've created myself (which are ultimately usually Objects in the JS world, but custom algorithms for get/set/sort and so on) and I want to render them differently in the console.

Another real-life example of it's usefulness is https://github.com/binaryage/cljs-devtools which formats Clojure/Script data structures into a more useful format when printing them to the console. It literally uses that feature: https://github.com/search?q=repo%3Abinaryage%2Fcljs-devtools...