When two adjacent 1-pixel-wide borders are collapsed together, the width of the resulting border is 2 pixels.
In CSS (Cascading Style Sheets), the border-width property specifies the width of an element's borders. When multiple borders are applied to the same element, the borders are placed on top of each other and the width of the resulting border is the sum of the individual border widths.
For example, if two adjacent borders are each 1 pixel wide, the resulting border will be 2 pixels wide. If the borders have different widths, the width of the resulting border will be the maximum of the individual border widths.