Well, you could insert those extra headers with JavaScript but it would not work on mobile devices. Probably better to combine the countries with the existing headers or use 2 separate questions.
Cheers,
Tony Partner Solutions, code and workarounds presented in these forums are given without any warranty, implied or otherwise.
Could you show me how to do it via Javascript (or to point me to a similar answer with javascript code) ? Because I do not need mobile device compatibility and I can't combine the countries with the header as I need to do more complex table as the attached table .
Your answer is OK to me . However I would like to proceed to a small adjustment in order to improve readability: I would like to insert a vertical line between each group ( so in your example between column 2 and 3).