Problem/Motivation
Drupal's theme system does not support tables with more than one row of table headers. Browsers support rendering multiple table headers and it can be useful for grouping/hierarchies of table header cells.
The idea of this feature request is to let users build table via the FAPI just as it was until then, but also to allow them to add one more depth level and let them build complex table header.
Further info:
http://jsfiddle.net/7pDqb/
http://stackoverflow.com/questions/18680044/how-can-i-construct-a-table-...
Proposed resolution
Update template_preprocess_table() method to let analyse headers with one more depth level, in a backward compatible way.
Update all table.html.twig files to reflect this change and support multiple rows of table headers.
Build a test to show the new feature and keep existing table test to show it's backward compatibility.
This would result in the ability to create this kind of headers via FAPI:
Remaining tasks
Design a solution and implement it.
User interface changes
none.
API changes
'#type' => 'table' FAPI element will support multiple row headers, that means the 'header' key would allow a one more depth level.
It may request another speical key to be achieved, but the exact API changes have to be discussed during implementation.