Q1. What units can border-width use?

px, em, rem, or keywords: thin, medium, thick.

Q2. How thick is thick keyword?

Approximately 4-5px, medium ~3-4px, thin ~1-2px (browser dependent).

Q3. How to set different widths per side?

Use border-width with 1-4 values: 2px 5px 8px 3px (top, right, bottom, left).

Q4. What happens if style not specified?

No border appears, regardless of width, because default style is none.

Q5. Can border-width be 0?

Yes, border-width: 0 removes border, same as border: none.