The Hidden Dangers of CSS Custom Properties in Shorthand

CSS custom properties (also known as CSS variables) offer a powerful way to manage colors, fonts, and other styling aspects within your website's design. They allow you to define reusable values, making your CSS more maintainable and organized. However, there's a subtle "gotcha" when using custom properties within shorthand properties that can lead to unexpected styling issues. This article explores this potential pitfall and provides clear guidance on how to avoid it.

Understanding the Problem

Shorthand properties in CSS allow you to set multiple style properties at once. For instance, padding lets you define top, right, bottom, and left padding simultaneously. Similarly, background allows you to combine properties like background-color, background-image, and others.

The issue arises when you try to use a custom property inside a shorthand property. If, for any reason, the custom property is invalid (e.g., it's misspelled or hasn't been defined yet), the entire shorthand declaration is invalidated. This means none of the styles within the shorthand property will be applied.

Example Breakdown

Let's illustrate this with a concrete example. Suppose you want to define a consistent spacing using a custom property:

:root {
  --main-spacing: 20px;
}

.my-element {
  padding: var(--main-spacing) 10px; /* Potential problem here */
}

In this scenario, we intend to set the top and bottom padding to var(--main-spacing) (20px) and the left and right padding to 10px. However, if --main-spacing is accidentally misspelled or not defined earlier in the CSS, the entire padding declaration will be ignored, and .my-element will have no padding at all.

A Safer Approach: Longhand Properties

The most reliable way to avoid this issue is to use longhand properties instead of shorthand when working with custom properties. Rewriting the previous example using longhand properties looks like this:

:root {
  --main-spacing: 20px;
}

.my-element {
  padding-top: var(--main-spacing);
  padding-bottom: var(--main-spacing);
  padding-left: 10px;
  padding-right: 10px;
}

Even if --main-spacing is invalid, the padding-left and padding-right declarations will still be applied correctly. Only the top and bottom padding will be affected.

Technical Deep Dive: The Cascade

This behavior stems from how the CSS cascade works. The cascade doesn't evaluate all properties simultaneously. It processes declarations one by one. When it encounters an invalid value within a shorthand property, it discards the entire declaration before applying other styles. By using longhand properties, we isolate the potential impact of an invalid custom property, ensuring that other styles are still applied as intended.

Practical Implications

This "gotcha" can be especially tricky to debug because the error might not be immediately obvious. If you're experiencing unexpected styling behavior, especially related to spacing or backgrounds, double-check your shorthand properties and consider switching to longhand if you're using custom properties.

Conclusion

CSS custom properties are a valuable tool for modern web development. However, using them within shorthand properties can lead to unexpected results due to how the cascade handles invalid values. By opting for longhand properties, you can prevent these issues and ensure that your styles are applied consistently and reliably.

Inspired by an article from https://css-tricks.com/maybe-dont-use-custom-properties-in-shorthand-properties/