A patch: one fix and one new export. ^0.3.0 picks it up.
A marshalled part now declares the Office extension namespaces it uses as ignorable Now each part root whose type allows mc:Ignorable adds the prefix for each extension namespace its content uses. Those roots are w:document, headers, footers, footnotes, endnotes, comments, styles, numbering, settings, fonts, worksheets, the workbook, tables, pivot parts and the comments-extension parts. This includes content the model holds as DOM. A list the part already has, as in any document Word wrote, is kept whole and in order; new prefixes are appended. Following Office's own practice, a namespace doesn't count if it's the root's own namespace, if it's used only inside an mc:Choice that requires it, or if it's used only inside an extension list's ext. Other roots are left alone.
New export IGNORABLE_EXTENSION_NAMESPACES: the namespaces the rule applies to. These are the twenty that Office itself lists in mc:Ignorable across the test corpus: Word's w14 w15 w16se w16cid w16 w16cex w16sdtdh w16sdtfl cr w16du wp14 and Excel's x14ac x15 x16r2 xr xr2 xr3 xr6 xr10 xr16. It lists namespaces only; each prefix comes from the prefix table in effect for the call.