I just did an experiment. I saw somewhere on one of the W3.CSS pages that you could download the css file and run it locally.
I examined the file and saw at the top two statements with html definitions. The first one included box-sizing:border-box. When I looked up the box-sizing property, the default was content-box, as you stated.
I commented out the two html statements and then added a link to that local file before the link to my css file. Amazingly my left, center, and right div blocks displayed as expected again. I did have to add "line-height: normal" to my p, th, td, and a fe