Table model loses extra cells in thead

[chaals]

This is https://www.w3.org/Bugs/Public/show_bug.cgi?id=22049

In the table model, there is an example of defining a thead with two rows. The first row ar th elements for the relevant columns. The second row are further useful information, which should be part of the headers unless there is a further contextual help or something - a sort of optional header, which we don't have.

The information should be provided as a header to the relevant cell in the body, but it is not picked up anywhere.

Possible fixes:

1. Change the example to use th for both rows
2. Change the algorithm to collect all cells in a thead as headers.

number 2 seems like a lot more work for implementors, so my suggestion is to do 1.

[chaals]

see also #819

[stevefaulkner]

The example in question http://w3c.github.io/html/tabular-data.html#example-52cbb871 is awkwardly constructed as the data cells in the thead should not be separate cells, instead their contents should be combined with the header cells in the first row. I think this example was (poorly) constructed to show use of td's in thead, I don't see the necessity for this. I have provided an example of the original table and a modified version, https://codepen.io/stevef/pen/JNYqPG