Custom text underlines

The text-decoration: underline CSS property provides insufficient control over the underline styling and position. While we wait for the CSS Text Decoration Module specification to become standard, we must rely on custom implementations.

My favorite approach is to use a linear-gradient to create an underline:

background-image: linear-gradient(gray, gray);
background-size: 100% 1px;
background-position: left bottom;
background-repeat: no-repeat;
Day by day, what you do is what you become.

Position

Position the underline by changing the vertical value of background-position:

background-position: left 1.05em;
Day by day, what you do is what you become.

Descenders

You'll notice that the underline overlaps the descenders of the text. By adding a text-shadow with a small offset to the right and left with the color of the background, you can hide the underline around descenders.

text-shadow: 0.1em 0 var(--background), -0.1em 0 var(--background);
Day by day, what you do is what you become.

Remember to set text-shadow: none in your ::selection rules.

Weight

Change the height of the background to increase the underline weight:

background-size: 100% 0.25em;
Day by day, what you do is what you become.

Dashes

By using a repeating-linear-gradient and leaving half the gradient transparent, you can customize a dashed underline:

background-image: repeating-linear-gradient(
  to right,
  var(--gray) 0%,
  var(--gray) 50%,
  transparent 50%,
  transparent 100%
);
background-size: 1ch 1px;
Day by day, what you do is what you become.

Change the horizontal value of background-size to modify the dash width:

background-size: 5ch 1px;
Day by day, what you do is what you become.

The ch unit is equal to the width of the "0" glyph in the current font, which can be useful for natural alignment.

Wrapping

Lastly, this approach also supports multi-line text:

Day by day,
what you do is what you become.

Let me know if you end up using this, or read more about other approaches in "Crafting link underlines on Medium."


Thanks to Franco for reminding me about this technique!