Remember how everyone was complaining how "heavy" all that glitz was? Images this, heavy HTML that, garishness this, load times that? I remember.
And then Windows 8 aesthetic came in, suddenly flat and a few colors with very reduced palette of available widgets became en vogue, but at the same time somehow the load and render times of sites bragging to be "simple" and "minimalistic" and "lightweight" soared with megabytes of assets none of your old sites sporting image borders and marble backgrounds ever had.
I think it includes smaller, denser text.
And something about the borders. Chunkier, more ornate, or griadients/light or something.