See documentation for the HTML 4.0 (XHTML) table model.
Description of one column of a table
See documentation for your XML editor or other software to determine the interaction between <colgroup> and <col>.
<!ELEMENT %col.qname; %col.content; >
This is an EMPTY element
<colgroup> Table Column Group (XHTML table model); <table> Table (XHTML table model)
See documentation for the HTML 4.0 (XHTML) table model.
htmltable.dtd