When I come to a website, it's usually because I want some particular piece of information (like movie times or ticket prices) or to have some particular interaction (to find new, interesting stories or to play a game). Good design (and good designers) keep that goal in mind, and support it by keeping distracting elements (possibly including weird, mis-styled scrollbars!) to a minimum, allowing me to focus.

Less-good designers just aim for making something more-or-less cohesive, and bad designers rarely aim higher than "something cool" whether or not it supports my goal. Mediocre and bad design hinders me from achieving my goal by hiding or obscuring basic tools of interaction like buttons or scroll-bars. Even good design can be a problem, if my goals do not align with what the designer had in mind (for example, the designer wants to make a good impression for first-time customers, but I'm trying to file a customer-support ticket).

I appreciate that many of the people commenting on that ticket are just trying to make the world a more pleasant place, but some times I wish browsers had a button that would blast away all the custom styling of a page and just display the content in a standardised fashion (like the Reader mode many browsers have, but not just limited to long-form articles).

If you want such a button, search for bookmarlets. I usually install one that makes all fonts the same (large) size and one that forces all texts to B/W. Can't live without it.