Inkscape <= 0.91 uses 90px per inch. Inkscape >= 0.92 uses SVG/CSS defined 96px per inch.
Work was done in 0.91 to make Inkscape produced drawings independent of which px per inch value is used but pre 0.91 files may still depend on 90 px per inch.
Files that have all the following properties:
- Pre-0.91 Inkscape files AND
- Which do not have a 'viewBox' AND
- A size (SVG root width/height) expressed in non-pixel units OR
- A size expressed in pixels that should be a fixed physical size OR
- Have grids with non-pixel units OR
- Have guides with non-pixel units OR
- Have font sizes with non-pixel units (pt).
If the a file is detected to have a problem, offer the user the options:
- To add a 'viewBox' that fixes the user unit to 90 px per inch.
- Switch the 'width' and 'height' attributes to an appropriate unit (i.e. 'mm' for A4)
- Adjust grids.
- Adjust guides.
- A 'viewBox' should NOT be automatically added.
- If a user does agree to add a 'viewBox' it should be undoable (Ctrl-Z) in case the user decides it was not appropriate.