org-html-table-header-tags

org-html-table-header-tags is a variable defined in `ox-html.el'.
Its value is

("" . "")

Documentation:
The opening tag for table header fields.
This is customizable so that alignment options can be specified.
The first %s will be filled with the scope of the field, either row or col.
The second %s will be replaced by a style entry to align the field.
See also the variable `org-html-table-use-header-tags-for-first-column'.
See also the variable `org-html-table-align-individual-fields'.

You can customize this variable.