What is a breakpoint in responsive Web design?
When a @media rule takes effect.
What is the advantage of using a percentage (e.g. 50%) width instead of a fixed (e.g. px) width for a “column” class in a responsive layout?
Percentages are more flexible.
If you introduce CSS rules for a smaller min-width after the styles for a larger min-width in your style sheet, the CSS rules for the smaller min-width will “win”. Why is that?
Because of cascading.