In Bootstrap 3, if a div is defined as <div class="col-md-3 col-lg-7">, how many columns will it take up in an xs viewport?

In Bootstrap 3, the grid system is based on a total of 12 columns. This means that any combination of column classes must equal 12 to fully utilize the grid within a row.

When a div is defined with classes col-md-3 and col-lg-7, those classes determine how many columns the div will occupy in specific viewport sizes. However, for extra small (xs) viewports, Bootstrap defaults to a full-width layout unless specified otherwise. This means that if no specific xs class is provided, the element will occupy all available 12 columns.

Since the div does not specify any classes for xs, it defaults to col-xs-12, which takes up all 12 columns available in the grid system. Therefore, the correct interpretation is that the div will take up 12 columns in an xs viewport, allowing it to span the entire width of the row.

This understanding is fundamental to the Bootstrap grid system, which is designed to be responsive and flexible across various screen sizes. The absence of designated column classes for smaller breakpoints means that the element will take up the maximum possible space allocated for xs, which is 12 columns.