In 9.1.7 <table:table-header-rows> we say:

-------------------------------------------------------------
A table shall not contain more than one <table:table-header-rows> element, except that
the table contains grouped rows defined by one or more <table:table-row-group> 9.1.9
elements.
-------------------------------------------------------------

This is not a correct sentence. Perhaps we mean:
-------------------------------------------------------------
A table shall not contain more than one <table:table-header-rows> element, unless
the table contains grouped rows defined by one or more <table:table-row-group> 9.1.9
elements.
-------------------------------------------------------------
or
-------------------------------------------------------------
A table shall not contain more than one <table:table-header-rows> element, except in the case that
the table contains grouped rows defined by one or more <table:table-row-group> 9.1.9
elements.
-------------------------------------------------------------

(Note that the section continues with conditions on this exception: "In this case, the table and ...")

The corresponding sentence occurs in 9.1.11 <table:table-header-columns>.