Hi Jim,
As I mentioned before, implementing a responsive grid inside plugin CSS would be quite redundant. If you are using a responsive theme, you already have a responsive grid implemented there, and adding another grid would just increase CSS bloat. One of our goals is to keep as light touch as possible, without adding any more css/javascript/etc than is strictly necessary.
As Stanislav mentioned above, we are adding a filter (and, probably a shortcode attribute, too) to allow people to add custom classes to Content Card containers. This will allow our users to leverage grid systems that are already in place in their themes. For example, if you use Bootstrap based theme, using a shortcode like this [contentcards url="https://example.com" class="col-sm-6 col-md-4 col-lg-3"]
will allow you to arrange content cards in a responsive grid fashion.
If your theme uses some other CSS framework, names of the classes would be different, but the general principle would still apply. I think this approach will allow our users to use Content Cards in grid view, while not increasing CSS bloat of our plugin.