Let spreadsheet text elements take the xml:space attribute, as the
Java officeotron 0.8.8 release patched its copy of this schema.
Spreadsheets in the wild carry the attribute on shared string and
inline string text.

--- a/sml.rnc
+++ b/sml.rnc
@@ -11,6 +11,7 @@
 namespace xdr =
   "http://schemas.openxmlformats.org/drawingml/2006/spreadsheetDrawing"
 
+include "xml.rnc"
 sml_CT_AutoFilter =
   attribute ref { sml_ST_Ref }?,
   element filterColumn { sml_CT_FilterColumn }*,
@@ -1982,10 +1983,10 @@
 sml_CT_PhoneticRun =
   attribute sb { xsd:unsignedInt },
   attribute eb { xsd:unsignedInt },
-  element t { s_ST_Xstring }
+  element t { xml_space?, s_ST_Xstring }
 sml_CT_RElt =
   element rPr { sml_CT_RPrElt }?,
-  element t { s_ST_Xstring }
+  element t { xml_space?, s_ST_Xstring }
 sml_CT_RPrElt =
   (element rFont { sml_CT_FontName }?
    | element charset { sml_CT_IntProperty }?
@@ -2003,7 +2004,7 @@
    | element vertAlign { sml_CT_VerticalAlignFontProperty }?
    | element scheme { sml_CT_FontScheme }?)+
 sml_CT_Rst =
-  element t { s_ST_Xstring }?,
+  element t { xml_space?, s_ST_Xstring }?,
   element r { sml_CT_RElt }*,
   element rPh { sml_CT_PhoneticRun }*,
   element phoneticPr { sml_CT_PhoneticPr }?