<td><br /></td> here <br /> what is doing ?

Answer 1

It's a linebreak instead of content to make table cells (<td>) visible. Try it in the Playground with and without <br>.

Answer 2

To see the effect of <br> you need to remove both <br> in the 2nd row of the table. Without it the cell size will shrink (no content) whereas with <br> the cell size will be kept (as if there were content). However only removing one <br> the cell size will still be larger from the call from the other column in the same row since the cell widths are constant across the row.

Answer 3

<td>