Variable / Dynamic number of cells in table row - always fill the width - html

Question

How can I make a table with multiple rows that have different number of columns (but not always the same number .... columns can be added and removed dynamically) but still have the cells in each row take the full width of the row?
Of course, I'd like to use only CSS and avoid a Javascript solution.
Here is an example of my issue:
table{
border: solid;
}
td{
background-color: black;
color: white;
}
.fullWidth{
/* width: 100%; doesn't work */
}
<table>
<tr>
<td>
cell 1
</td>
<td>
cell 2
</td>
<td>
cell 3
</td>
<td>
cell 4
</td>
<td>
Cells are added and removed dynamically
</td>
</tr>
<tr>
<td class='fullWidth'>
this cell should be the full width of the table
</td>
</tr>
</table>
EDIT: I was trying to keep the question short and to the point, so I left out most of the details. :P
I am using a table because I am using Bootstrap's responsive tables on a jQuery Datatable, and I need to insert a custom row to add details to some of the rows. (Bootstrap's responsive tables already utilizes the default "child" row feature of Datatables, or I would have just used that)

Answer 1

Judging by the comments, what I'm asking isn't possible with CSS alone. I'm going to open a new question to address the Javascript solution with bootstrap responsive tables.
So the answer to the question on this page is: no, it is not possible.
EDIT: as long as you set the colspan correctly on the first inserted row, it looks like bootstrap's responsive table takes care of the rest. 👍