I think the way to go is for the site author to offer their own content style. Then the user has the choice to use that, or to use their own style. They can default to whatever they want (theirs or sites'), and override on a case-by-case basis.
If a user is determined to view your site with different colors/fonts, then there's no reason to fight them. They could even have very legitimate reasons (poor eyesight needing bigger fonts, color blindness issues, etc.)
But for this to really work, we need to drastically decrease the variation involved in the site markup (HTML), and rely much more heavily on the styling (CSS) for page layout. HTML5 semantic elements don't go nearly far enough.
There is a reason to fight them: layout. The battle is not developers vs users, its designers vs users. Designers want everything to look exactly like their vision (understandably), and that means things like bits of text fitting in exactly the right dimensions and placement. Once you let users resize things on a whim, that goes out the window, because most likely everything will look completely wrong.
It also opens the door for dead simple ad blocking that can't be stopped, and business owners will never stand for that. The purpose that the web has been bent to in the last 15 years is simply not compatible with user styles.
If a user is determined to view your site with different colors/fonts, then there's no reason to fight them. They could even have very legitimate reasons (poor eyesight needing bigger fonts, color blindness issues, etc.)
But for this to really work, we need to drastically decrease the variation involved in the site markup (HTML), and rely much more heavily on the styling (CSS) for page layout. HTML5 semantic elements don't go nearly far enough.