font-weight

The font-weight property selects the weight of the font.

Description

Values	normal \| bold \| bolder \| lighter \| 100 \| 200 \| 300 \| 400 \| 500 \| 600 \| 700 \| 800 \| 900 \| inherit
Initial value	normal
Applies to	All elements
Inherited	Yes

bolder, lighter
The 'bolder' and 'lighter' values select font weights that are relative to the weight inherited from the parent.

100 to 900
Specifies from thin to thick characters. The keyword 'normal' is synonymous with '400', and 'bold' is synonymous with '700'.

inherit
Takes the same specified value as the property for the element's parent.

[style.css]

p#no{ font-weight: normal; }
p#b{ font-weight: bold; }

[index.html]

<body>
  <p id="no">This is a paragraph(normal)</p>
  <p id="b">This is a paragraph(bold)</p>
</body>

The CSS specification defines the font-style property in 15.6 Font boldness.