auto

The browser chooses an appropriate width for the text decoration line.

from-font

If the font file includes information about a preferred thickness, use that value. If the font file doesn't include this information, behave as if auto was set, with the browser choosing an appropriate thickness.

<length>

Specifies the thickness of the text decoration line as a <length>, overriding the font file suggestion or the browser default.

<percentage>

Specifies the thickness of the text decoration line as a <percentage> of 1em in the current font. A percentage inherits as a relative value, and so therefore scales with changes in the font. The browser must use a minimum of 1 device pixel. For a given application of this property, the thickness is constant across the whole box it is applied to, even if there are child elements with a different font size.

text-decoration-thickness = 
  auto          |
  from-font     |
  <length>      |
  <percentage>  

HTML

<p class="thin">Here's some text with a 1px red underline.</p>
<p class="thick">This one has a 5px red underline.</p>
<p class="shorthand">This uses the equivalent shorthand.</p>

CSS

.thin {
  text-decoration-line: underline;
  text-decoration-style: solid;
  text-decoration-color: red;
  text-decoration-thickness: 1px;
}

.thick {
  text-decoration-line: underline;
  text-decoration-style: solid;
  text-decoration-color: red;
  text-decoration-thickness: 5px;
}

.shorthand {
  text-decoration: underline solid red 5px;
}

Results

• text-decoration
• text-underline-offset