text-underline-position

The text-underline-position CSS property specifies the position of the underline which is set using the text-decoration property's underline value.

auto

The user agent uses its own algorithm to place the line at or under the alphabetic baseline.

from-font

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

under

Forces the line to be set below the alphabetic baseline, at a position where it won't cross any descenders. This is useful for ensuring legibility with chemical and mathematical formulas, which make a large use of subscripts.

left

In vertical writing-modes, this keyword forces the line to be placed on the left side of the text. In horizontal writing-modes, it is a synonym of under.

right

In vertical writing-modes, this keyword forces the line to be placed on the right side of the text. In horizontal writing-modes, it is a synonym of under.

auto-pos

A synonym of auto, which should be used instead.

above

Forces the line to be above the text. When used with East-Asian text, using the auto keyword will lead to a similar effect.

below

Forces the line to be below the text. When used with alphabetic text, using the auto keyword will lead to a similar effect.

auto | from-font | [ under || [ left | right ] ]

Let's take a couple of simple example paragraphs:

<p class="horizontal">Lorem ipsum dolor sit amet, consectetur adipiscing elit.
Nullam consectetur ac turpis vel laoreet. Nullam volutpat pharetra lorem, sit
amet feugiat tortor volutpat quis. Nam eget sodales quam. Aliquam accumsan
tellus ac erat posuere.</p>

<p class="vertical">Lorem ipsum dolor sit amet, consectetur adipiscing elit.
Nullam consectetur ac turpis vel laoreet. Nullam volutpat pharetra lorem, sit
amet feugiat tortor volutpat quis. Nam eget sodales quam. Aliquam accumsan
tellus ac erat posuere.</p>

Our CSS looks like this:

p {
  font-size: 1.5rem;
  text-transform: capitalize;
  text-decoration: underline;
  text-decoration-thickness: 2px;
}

.horizontal {
  text-underline-position: under;
}

.vertical {
  writing-mode: vertical-rl;
  text-underline-position: left;
}

In this example we set both the paragraphs to have a thick underline. In the horizontal text we use text-underline-position: under; to put the underline below all the descenders.

In the text with a vertical writing-mode set, we can then use values of left or right to make the underline appear on the left or right of the text as required.

The live example looks like this:

Because the text-underline-position property inherits and is not reset by the text-decoration shorthand property, it may be appropriate to set its value at a global level. For example, the under value may be appropriate for a document with lots of chemical and mathematical formulas, which make a large use of subscripts.

:root {
  text-underline-position: under;
}

• The text-decoration property is a shorthand for setting most text-decoration properties, including text-decoration-line, text-decoration-color, and text-decoration-style. However, it does not set text-underline-position.
• Microsoft non-standard values documentation