Adding a progress bar to the top of the page is a simple way of showing the user how far down the page they have read. There is a scroll bar on the right hand side of the page on my current device, but that isn't always present on all devices.
I was working on a user dashboard recently and decided to add some more signposting to values being updated. Users were able to select different parameters in order to produce different values, but I thought that the values being updated weren't clear enough.
When I first heard the term "Shadow DOM" I had to laugh at the name. I had visions of a cloak and dagger HTML element that would only be seen on the dark web. Or some sort of nefarious shadow site that was the after effect of a hack.