Each cell can be assigned zero or more header cells. The algorithm for assigning header cells to a cell principal cell is as follows.

1. Let header list be an empty list of cells.

2. Let (principal_x, principal_y) be the coordinate of the slot to which the principal cell is anchored.

3. If the principal cell has a headers attribute specified

   1. Take the value of the principal cell's headers attribute and split it on ASCII whitespace, letting id list be the list of tokens obtained.

   2. For each token in the id list, if the first element in the Document with an ID equal to the token is a cell in the same table, and that cell is not the principal cell, then add that cell to header list.

   If principal cell does not have a headers attribute specified

   1. Let principal_width be the width of the principal cell.

   2. Let principal_height be the height of the principal cell.

   3. For each value of y from principal_y to principal_y+principal_height-1, run the internal algorithm for scanning and assigning header cells, with the principal cell, the header list, the initial coordinate (principal_x, y), and the increments Δx=−1 and Δy=0.

   4. For each value of x from principal_x to principal_x+principal_width-1, run the internal algorithm for scanning and assigning header cells, with the principal cell, the header list, the initial coordinate (x, principal_y), and the increments Δx=0 and Δy=−1.

   5. If the principal cell is anchored in a row group, then add all header cells that are row group headers and are anchored in the same row group with an x-coordinate less than or equal to principal_x+principal_width-1 and a y-coordinate less than or equal to principal_y+principal_height-1 to header list.

   6. If the principal cell is anchored in a column group, then add all header cells that are column group headers and are anchored in the same column group with an x-coordinate less than or equal to principal_x+principal_width-1 and a y-coordinate less than or equal to principal_y+principal_height-1 to header list.

4. Remove all the empty cells from the header list.

5. Remove any duplicates from the header list.

6. Remove principal cell from the header list if it is there.

7. Assign the headers in the header list to the principal cell.