Table with 100% width but relative cells?

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I'm making a table, and what I want is to have the whole table be 100%
width with relative cells.
I have rows that are headers, where it spans all columns, and I want
that to fit the whole page. But then, I want all of the non-header rows
to have the cells fit relatively. Right now, I have a fixed width on
the cell, but the problem is the cell has dynamic content, so I don't
want to have to monitor the table every time data in it changes to make
sure it still fits everything OK.
I must admit, I don't know CSS, I am just going off of straight HTML,
but if I need CSS to do it that would be fine.
Any help would be much appreciated.

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