Created attachment 76124 [details]
Sample file (details of how to create it also in description)

Impress saves invalid ODF markup in tables in certain circumstances.

Repro:

1. Create a new presentation in Impress
2. Insert a table (size doesn't matter)
3. Save as ODP

Unpack the file. Inside \styles.xml is:


    <table:table-template text:style-name="default">
      <table:first-row text:style-name="gray3"/>
      ...

This is invalid markup according to ODF 1.2. In the ODF 1.2 schemas,
text:style-name cannot live in table:first-row (it should be table:style-name
instead):

       <define name="table-first-row">
              <element name="table:first-row">
                     <ref name="common-table-template-attlist"/>
                     <empty/>
              </element>
       </define>

       ...

       <define name="common-table-template-attlist">
              <attribute name="table:style-name">
                     <ref name="styleNameRef"/>
              </attribute>
              <optional>
                     <attribute name="table:paragraph-style-name">
                           <ref name="styleNameRef"/>
                     </attribute>
              </optional>
       </define>


Suggested fix: Use table:style-name inside tables instead.

I have also filed this as a LibreOffice bug (https://bugs.freedesktop.org/show_bug.cgi?id=35190).