Given this rule: div{ border-width:10px 3px 5px 9px; } What is the width of the left border?

In the given rule for the CSS property border-width, the values are specified in the order of top, right, bottom, and left borders. Therefore, when you see the syntax border-width: 10px 3px 5px 9px;, the first value corresponds to the top border, the second to the right border, the third to the bottom border, and the fourth value is for the left border.

Thus, the value for the left border is represented by the last number, which in this case is 9px. Therefore, the correct width of the left border is 9 pixels.