HTML tables use <th> to indicate a table header and <td> to indicate a table cell - these cells should be wrapped in a row tag (<tr>) Table headers should be wrapped in the <theader> tags and the other content cells should be wrapped in the <tbody> tags. This is used mainly for readability.
To have a shared header (a header that spans multiple subheaders), they need to be put in a new row. You then give it the property of rowspan to span multiple columns (as many as needed).