xmllint.exe --xpath " //header[ contains(.,'rationale') ]/node()[ following-sibling::l or self::l]   " fudge

                   1. Purpose and rationale
                   <p>purpose 1</p>
                   <p>Purpose 2</p>
                   <p>purpose 3</p>
                   <l>
                      <li>purpose list 1</li>
                      <li>list2</li>
                    </l>
                    2. Purpose and rationale
                    <p>2 purpose 1</p>
                    <p>2 Purpose 2</p>
                    <p>2 purpose 3</p>
                    <l>
                        <li>2 purpose list 1</li>
                        <li>2 list2</li>
                    </l>