Seeing as how most people only use one browser (data collected out of thin air), wouldn't changing the native styling on a browser scrollbar be confusing for people expecting the native styling?
Assuming you were using Firefox, what would you think about those two white scrollbars that stick out like a sore thumb in a dark interface?
Secondly, I prefer it. Far too many websites adjust things to the point where it's impossible to actually see. (Case in point: link styling. To the point where I have bookmarklets to remove styling from pages.)
I understand having the option to do so. But at the same time, I wouldn't use said option. To me, readability > aesthetics. And far too many websites don't respect that.
Please read JS;DR: http://tantek.com/2015/069/t1/js-dr-javascript-required-dead
You need to start differenciating between "documents" and "applications". What GP linked is a full-blown application. There's no "non-javascript" version of it.
I have been watching comments here and it is absolutely infuriating how it's very obviously armchair designers with zero experience in the field who are yelling out "aesthetics don't matter, scrollbars should always look like my system's scrollbars".
First of all, that is a ridiculous statement. On Linux for example there is no "system scrollbars", the styling is up to the toolkit and it is different in every single web browser out there. There's no consistency to be had across applications.
Second of all, every other toolkit element can be styled. Buttons. Dropdown menus. Text fields. Everything. They can all be styled not so your "documents" will be less readable, but so that developers and designers are empowered to create applications that look good, feel fluid, native, consistent.
They can be styled because applications can be more than just 20-input field forms. Native applications have the power to style their scrollbars (and they do so all the time), so web applications need it too to match such capabilities. We're not talking about an unused feature here.
And to those complaining about scrolling behaviour, accessibility etc: Those are the exact reasons why we need scrollbar styling. Because when your company is hounding you to have scrollbars that don't look out of place on a major browser "and look, our competition does it", you're realistically not just going to tell them "well, uh, readability is important". You're going to use fake, pure-js scrollers and accessibility will suffer. Everybody loses.
This isn't about purple scrollbars for your text documents.
Is it a full-blown application? Sure, I guess. But I could care less about that.
The actual content is a couple of screenshots - not exactly something that requires JS.
Also, I'm not speaking about designers at all. I am not a website designer, and never claimed to be one.
I'm speaking as someone who uses a web browser. As I said earlier: To me, readability > aesthetics. Especially as I'm often on-the-go. Things that look better but are lower contrast are often unreadable when you've got any amount of glare on the screen.
I use one web browser for the majority of my browsing. So yes, there is consistency. I'm not talking about across applications, I'm talking about within an application.
It may not be about purple scrollbars for your text documents, but that's what it will largely end up being used for.
And yes, native applications have a greater power to style things. But there is a distinction. Native applications inherently require greater trust. I'm not going to install any random application that comes my way.
Also, there are applications I stay away from precisely because of that - precisely because they have weird styling, and weird UIs. Case in point: Github's desktop application. Great program - or would be if I could get over the UI. But as is, I don't tend to use it.
That being said, it's a bit of a moot point regardless. It's just yet another thing I'll add to the list of bookmarklets to disable things to make websites readable. Along with many of the other things you mention as people being able to style as features.
Taking away freedoms from developers should only ever be done for security reasons. Not because you're scared they'll "misuse it and make ugly apps". Who cares about "ugly apps"? They are weeded out over time by natural selection.