"Responsive" table with columns that don't collapse
"Responsive" table with columns that don't collapse

I have a very simple problem: my supposedly responsive tables do not have columns which collapse. The resulting table doesn't fit for narrower resolutions. I've triggered everything properly and set the table width to 100% but still having problems.
I recently removed the 'width="100%"' HTML attribute from the opening table tag and believe this to be part of the problem. I did this because it was showing as invalid HTML as the attribute has been depreciated in favor of CSS.
Is there a way to fix the problem without this HTML attribute? Why isn't the responsive table working? Here's the page: https://fliint.org/hsc-modern-history/. Resize the page and see for yourself.
Thank you.