It’s the opposite: if you insist on keeping your browser maximized that can mean that you probably need all the width on some other site which scales poorly.

I never keep my browser maximized.

As a reference, I maximize my browsers when I'm using developers tools or when I'm using pseudo desktop web apps, usually customer mandated. I'm thinking about Kanbanize, Trello, almost all of Google, Travis, etc. Apps that display a lot of information.

HN and other text based sites are more readable in narrower windows or on a phone.

HN on a phone suffers when comments are deeply nested (this one) and when somebody quotes text using spaces as of it was a block of code.

Nope, problem is each window may need a different sizing. With a tabbed interface you resize the lot.

For those with your use cases the browsers should allow “per site” saved local margins.

Bur it seems the ad delivery has the priority.

I simply have a separate browser window for “wide development” sites. I see nothing wrong with that.

Tree-Style-Tabs (Firefox extension) prevents this issue.

Just tried out Tree style tabs, how does this prevent this issue? I have two pages in two tabs open. To read one I resize the window, I go to the other it has been resized also.