Table heading group
The thead element may optionally be used with a table in order to group together a set of rows that describe the contents of each table column.
The thead element may be the target of a CSS selector, simplifying the styling for all of the data cells it contains.
The thead element should be made up of rows whose cells are defined using "th" elements (rather than "td" elements).
When both colgroup and thead elements are used, the colgroup element must be placed first.