Q1. What color formats can border-color use?

Any valid color: names, hex, RGB, RGBA, HSL.

Q2. How to set different colors per side?

Use border-color with 1-4 values: red green blue orange (top, right, bottom, left).

Q3. How to make semi-transparent border?

Use RGBA: border: 3px solid rgba(0,0,0,0.5);

Q4. How to change border color on hover?

.btn:hover { border-color: blue; }

Q5. Can border-color be used in form validation?

Yes, style :valid with green border, :invalid with red border for visual feedback.