normal

The normal letter spacing for the current font. Unlike a value of 0, this keyword allows the user agent to alter the space between characters in order to justify text.

<length>

Specifies extra inter-character space in addition to the default space between characters. Values may be negative, but there may be implementation-specific limits. User agents may not further increase or decrease the inter-character space in order to justify text.

A large positive or negative letter-spacing value will make the word(s) the styling is applied to unreadable. For text styled with a very large positive value, the letters will be so far apart that the word(s) will appear like a series of individual, unconnected letters. For text styled with a very large negative value, the letters will overlap each other to the point where the word(s) may be unrecognizable.

Legible letter-spacing must be determined on a case-by-case basis, as different font families have different character widths. There is no one value that can ensure all font families automatically maintain their legibility.

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

Some written languages should not have any letter spacing applied. For instance, languages that use the Arabic script expect connected letters to remain visually connected, as in the following example. Applying letter spacing will lead the text to look broken.

شسيبتنمك

letter-spacing = 
  normal    |
  <length>  

HTML

<p class="normal">letter spacing</p>
<p class="em-wide">letter spacing</p>
<p class="em-wider">letter spacing</p>
<p class="em-tight">letter spacing</p>
<p class="px-wide">letter spacing</p>

CSS

.normal {
  letter-spacing: normal;
}
.em-wide {
  letter-spacing: 0.4em;
}
.em-wider {
  letter-spacing: 1em;
}
.em-tight {
  letter-spacing: -0.05em;
}
.px-wide {
  letter-spacing: 6px;
}

Result

• font-kerning
• word-spacing