I'm pretty sure layout reflows were considered, as HTML rendering itself is incremental in almost all sensible browsers.

(Thus the ideas of embedding CSS and JS in localized pieces where applicable.)

Scroll gestures were not a thing typically, you just did a full request.

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.