Darkle YouTube uses custom thin bars, do you think it offers a poor user experience?
They only use it on specific elements (in Firefox), but yes it degrades the experience and they should respect the device setting. If I wanted thin scrollbars I'd set it in the system settings.
If I open up Youtube in Chrome I can see what you mean - yes it's completely stupid and ugly to force that on the user - no thanks.
Darkle I have to say that if the only reason is the width of the scrollbar I have bad news for you, because very soon browsers on Windows will start to force the new WinUI scrollbars, in fact, you can enable them from now on in edge://flags/ by enabling "Windows style overlay scrollbars." which by default are thin, this was happening on macOS for quite some time now, then I imagine you will start to hate the system bars too, so in this case I don't see a possible solution...
What Microsoft does with Edge doesn't affect any other browser, and no the system doesn't force thin scrollbars - you have complete control in Windows over not only which style you want but precisely what thickness you want and whether they are always visible or not it's a registry setting or you can use Winaero Tweaker. Webmasters set thin scrollbars because they think it looks good, but bare in mind they were doing this partly because they didn't have any styling control over the colour of the scrollbars which isn't the case any more. That you think you know what the user wants better than the user themselves is precisely what leads to a poor user experience:


Overriding what the user expressly wants is a bad idea. The only thing in my opinion that is reasonable to override with a thin scrollbar is a textarea, but even then I think the benefit does not outweigh the annoyance it comes with and you should just work with the user's default preference as set at the system level. The problem with setting thin scrollbars is that it doesn't read the system setting, which in my case as you see is set to the same size as the standard bar effectively disabling the stupid thin scrollbars, instead the browser be it Firefox or Chrome sets its own setting for thin bars that don't respect the system setting.