Comment 23 for bug 1239682

>> I think that if there's no viewbox it makes sense supposing that the file comes from the historical standard of 90 DPI.

unfortunately, if we were to do that, we would be breaking compatibility with external viewers which may not use a viewbox and may not be using 90 dpi as a standard. For example attached is a demo file. This was created in Inkscape 0.91 using a standard letter sized page, 8.5x11 in. Then I saved it as plain svg and deleted the viewbox to make it appear that it was created by a third-party software. This file loads correctly into recent Inkscape trunk, meaning that the ellipse fits the page border exactly. It also loads correctly into Internet Explorer 8, which is actually using the Adobe SVG Viewer 3.0. It also loads correctly into IE9 which has a native svg viewer. But it does not load correctly into Inkscape 0.91, due to the difference in resolution.
    I believe the 90 dpi issue will need to be dealt with separately, possibly with a Python extension like DPISwitcher from Bug 1389723