In my recent article about CSS underline bugs in Chrome, I discussed text-decoration-thickness and text-underline-offset, two relatively new and widely-supported CSS properties that give us more control over the styling of underlines.

Let me demonstrate the usefulness of text-decoration-thickness on a simple example. The Ubuntu web font has a fairly thick default underline. We can make this underline thinner like so:

:any-link { text-decoration-thickness: 0.08em;

Showing two links, a default and one that decreases the text-decoration-thickness.

/explanation Throughout this article, I will use the :any-link selector instead of the a element to match hyperlinks. The problem with the a tag as a selector is that it matches all <a> elements, even the ones that don’t have a href attribute and thus aren’t hyperlinks. The :any-link selector only matches <a> elements that are hyperlinks. Web browsers also use :any-link instead of a in their user agent stylesheets.

Hover underlines

Many websites, including Google Search and Wikipedia, remove underlines from links and only show them when the user hovers a link. Removing underlines from links in body text is not a good idea, but it can make sense in places where links are more spaced apart (navigation, footer, etc.). With that being said, here’s a simple implementation of hover underlines for links in the website’s header:

header :any-link { text-decoration: none;
} header :any-link:hover { text-decoration: underline;

But there’s a problem. If we tested this code in a browser, we’d notice that the underlines in the header have the default thickness, not the thinner style that we declared earlier. Why did text-decoration-thickness stop working after we added hover underlines?

Let’s look at the full CSS code again. Can you think of a reason why the custom thickness doesn’t apply to the hover underline?

:any-link { text-decoration-thickness: 0.08em;
} header :any-link { text-decoration: none;
} header :any-link:hover { text-decoration: underline;

The reason for this behavior is that text-decoration is a shorthand property and text-decoration-thickness its associated longhand property. Setting text-decoration to none or underline has the side effect of re-initializing the other three text decoration components (thickness, style, and color). This is defined in the CSS Text Decoration module:

The text-decoration property is a shorthand for setting text-decoration-line, text-decoration-thickness, text-decoration-style, and text-decoration-color in one declaration. Omitted values are set to their initial values.

You can confirm this in the browser’s DevTools by selecting one of the hyperlinks in the DOM inspector and then expanding the text-decoration property in the CSS pane.

DevTools screenshot showing text-decoration styles on the :any-link pseudo-selector.

In order to get text-decoration-thickness to work on hover underlines, we’ll have to make a small change to the above CSS code. There are actually multiple ways to achieve this. We could:

  • set text-decoration-thickness after text-decoration,
  • declare the thickness in the text-decoration shorthand, or
  • use text-decoration-line instead of text-decoration.

Choosing the best text-decoration option

Our first thought might be to simply repeat the text-decoration-thickness declaration in the :hover state. It’s a quick and simple fix that indeed works.

/* OPTION A */ header :any-link { text-decoration: none;
} header :any-link:hover { text-decoration: underline; text-decoration-thickness: 0.08em; /* set thickness again */

However, since text-decoration is a shorthand and text-decoration-thickness is its associated longhand, there really should be no need to use both at the same time. As a shorthand, text-decoration allows setting both the underline itself and the underline’s thickness, all in one declaration.

/* OPTION B */ header :any-link { text-decoration: none;
} header :any-link:hover { text-decoration: underline 0.08em; /* set both line and thickness */

If this code looks unfamiliar to you, that could be because the idea of using text-decoration as a shorthand is relatively new. This property was only subsequently turned into a shorthand in the CSS Text Decoration module. In the days of CSS 2, text-decoration was a simple property.

Unfortunately, Safari still hasn’t fully caught up with these changes. In the WebKit browser engine, the shorthand variant of text-decoration remains prefixed (-webkit-text-decoration), and it doesn’t support thickness values yet. See WebKit bug 230083 for more information.

This rules out the text-decoration shorthand syntax. The above code won’t work in Safari, even if we added the -webkit- prefix. Luckily, there’s another way to avoid repeating the text-decoration-thickness declaration.

When text-decoration was turned into a shorthand, a new text-decoration-line longhand was introduced to take over its old job. We can use this property to hide and show the underline without affecting the other three text decoration components.

/* OPTION C */ header :any-link { text-decoration-line: none;
} header :any-link:hover { text-decoration-line: underline;

Since we’re only updating the line component of the text-decoration value, the previously declared thickness remains intact. I think that this is the best way to implement hover underlines.

Be aware of shorthands

Keep in mind that when you set a shorthand property, e.g., text-decoration: underline, any missing parts in the value are re-initialized. This is also why styles such as background-repeat: no-repeat are undone if you set background: url(flower.jpg) afterwards. See the article “Accidental CSS Resets” for more examples of this behavior.

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