After adding some margins and changing the orientation of printer:

printer.setPageMargins(QMarginsF(25, 60, 25, 60));
printer.setPageOrientation(QPageLayout::Landscape);

the result is stretched.

Another case, when the CSS of the web content has

@page {
    size: letter landscape;
}

and the provided QPrinter has been configured to portrait, the result is similar.

Even worse case where the pages are not uniform:

@page :left {
  size: landscape;
}
@page :right {
  size: portrait;
}