From invalid to valid SVG: Inkscape-files
To make Inkscape-SVG-files valid according to the guidelines of W3C and its validator: http://validator.w3.org/#validate_by_upload, these steps are required and work every time:
1. Load the SVG-file into Inkscape
- a) Select all elements and then ungroup all the elements of the picture by pressing the ungroup button several times (or by pressing CTRL+SHITFT+G) till even the words are ungrouped and every letter becomes a single path.
- b) Goto File/Vacuum Defs and run it. This will declutter the SVG-file.
2. It might be that the file is still not W3C-valid. One reason can be the text in the SVG-file. A quick and dirty solution is to press Path/Object to Path with all the elements of SVG-file still selected. All the elements of the SVG-file will be paths now.
- a) Regroup all the elements of the file by pressing the group-button or by pressing CTRL+G.
3. Save the file as Plain SVG. (There is no consensus on this. For the pros and cons on this, see this page: https://commons.wikimedia.org/wiki/Help:Inkscape#Bugs_and_workarounds).
If this still hasn't worked, you can open the file in a text-editor. SVG-files consist of XML-code and this code can be worked upon.