Measure two is empty;
it only contains a time signature but not anything else. Some
applications use this approach to insert time signatures for purely
visual reasons, for example, displaying a final time signature at the
end of a line, followed by a forced line break.
While doing so is valid MusicXML, having measures with a zero length
is semantically questionable. In particular, if the displayed bar
number is derived from ‘<measure number="...">’ (as it usually
is), such empty measures make the bar numbers being off.
Additionally, it is not portable, since other applications ‘rectify’
it by displaying this empty measure, giving it a default
length.MusicXML Part1G2D411quarterE411quarterF411quarterG411quarterA411quarterA411quarterA411quarter