mediaBox

Page box in PDF user-space units (1/72 inch). left, bottom, right, top.

ISO 32000-1, 7.7.3.3 makes /MediaBox required and inheritable. A damaged page with no usable box, whether missing, shorter than four numbers or empty, gets a US Letter box of 612 x 792 with a warning, as MuPDF does, so the page still shows (#330).