font-weight: 
normal | bold | bolder | lighter |
100 | 200 | 300 | 400 | 500 | 600 | 700 | 800 | 900 | inherit

• normal is equivalent to 400
• bold is equivalent to 700
• bolder is relative to parent's weight
• lighter is relative to parent's weight
• Browser may give same results for, say, 100, 200 and 300

normal bold
Parent and bolder
Parent and lighter
100 200 300 400 500 600 700 800 900

Variable support in browsers and depends on font variants available and whether browser is smart enough to use them

Do not rely on more than two levels of boldness