Sure, it's incremental, but static site layouts, especially the old float-based ones will have had their headers and sidebars loaded from the start and the main

Sure, it's incremental, but static site layouts, especially the old float-based ones will have had their headers and sidebars loaded from the start and the main content would not jump around. Modern pages with ads and widgets popping in potentially anywhere main remain unusable and unreadable because the main content keeps jumping around.