When I am browsing, I don’t want to have to care.
When I am designing a site, I don’t want the user to have to care.
CSS is my enemy
How items flow with CSS is a property of the object instead of its container.
Now, maybe I am being a bit harsh in calling it a fundamental flaw. For all I know, it was a conscious choice. My guess though is that it seemed natural to people who wrote browsers in the early 90’s (<blink> tags and all).