/* Keyword values */
font-weight: normal;
font-weight: bold;

/* Keyword values relative to the parent */
font-weight: lighter;
font-weight: bolder;

/* Numeric keyword values */
font-weight: 100;
font-weight: 200;
font-weight: 300;
font-weight: 400; /* normal */
font-weight: 500;
font-weight: 600;
font-weight: 700; /* bold */
font-weight: 800;
font-weight: 900;

/* Global values */
font-weight: inherit;
font-weight: initial;
font-weight: revert;
font-weight: revert-layer;
font-weight: unset;

The font-weight property is specified using any one of the values listed below.

normal

Normal font weight. Same as 400.

bold

Bold font weight. Same as 700.

lighter

One relative font weight lighter than the parent element. Note that only four font weights are considered for relative weight calculation; see the Meaning of relative weights section below.

bolder

One relative font weight heavier than the parent element. Note that only four font weights are considered for relative weight calculation; see the Meaning of relative weights section below.

<number>

A <number> value between 1 and 1000, inclusive. Higher numbers represent weights that are bolder than (or as bold as) lower numbers. Certain commonly used values correspond to common weight names, as described in the Common weight name mapping section below.

In earlier versions of the font-weight specification, the property accepts only keyword values and the numeric values 100, 200, 300, 400, 500, 600, 700, 800, and 900; non-variable fonts can only really make use of these set values, although fine-grained values (e.g. 451) will be translated to one of these values for non-variable fonts using the Fallback weights system.

CSS Fonts Level 4 extends the syntax to accept any number between 1 and 1000 and introduces Variable fonts, which can make use of this much finer-grained range of font weights.

If the exact weight given is unavailable, then the following rule is used to determine the weight actually rendered:

• If the target weight given is between 400 and 500 inclusive:
  • Look for available weights between the target and 500, in ascending order.
  • If no match is found, look for available weights less than the target, in descending order.
  • If no match is found, look for available weights greater than 500, in ascending order.
• If a weight less than 400 is given, look for available weights less than the target, in descending order. If no match is found, look for available weights greater than the target, in ascending order.
• If a weight greater than 500 is given, look for available weights greater than the target, in ascending order. If no match is found, look for available weights less than the target, in descending order.

When lighter or bolder is specified, the below chart shows how the absolute font weight of the element is determined.

Note that when using relative weights, only four font weights are considered — thin (100), normal (400), bold (700), and heavy (900). If a font-family has more weights available, they are ignored for the purposes of relative weight calculation.

The numerical values 100 to 900 roughly correspond to the following common weight names (see the OpenType specification):

Most fonts have a particular weight which corresponds to one of the numbers in Common weight name mapping. However some fonts, called variable fonts, can support a range of weights with a more or less fine granularity, and this can give the designer a much closer degree of control over the chosen weight.

For TrueType or OpenType variable fonts, the "wght" variation is used to implement varying widths.

People experiencing low vision conditions may have difficulty reading text set with a font-weight value of 100 (Thin/Hairline) or 200 (Extra Light), especially if the font has a low contrast color ratio.

• MDN Understanding WCAG, Guideline 1.4 explanations
• Understanding Success Criterion 1.4.8 | W3C Understanding WCAG 2.0

font-weight = 
  <font-weight-absolute>  |
  bolder                  |
  lighter                 

<font-weight-absolute> = 
  normal             |
  bold               |
  <number [1,1000]>  

HTML

<p>
  Alice was beginning to get very tired of sitting by her sister on the bank,
  and of having nothing to do: once or twice she had peeped into the book her
  sister was reading, but it had no pictures or conversations in it, "and what
  is the use of a book," thought Alice "without pictures or conversations?"
</p>

<div>
  I'm heavy<br />
  <span>I'm lighter</span>
</div>

CSS

/* Set paragraph text to be bold. */
p {
  font-weight: bold;
}

/* Set div text to two steps heavier than
   normal but less than a standard bold. */
div {
  font-weight: 600;
}

/* Set span text to be one step lighter
   than its parent. */
span {
  font-weight: lighter;
}

Result

• font-family
• font-style
• Fundamental text and font styling