CSS Font Feature Values: Mastering OpenType Feature Control

In the dynamic world of web design, typography plays a pivotal role in conveying brand identity, enhancing readability, and creating compelling visual experiences. While basic font styling is straightforward, achieving sophisticated typographic effects often requires delving into the advanced capabilities of OpenType fonts. Fortunately, CSS provides powerful tools to harness these capabilities through font-feature-values. This comprehensive guide will explore how to leverage these CSS features to gain granular control over OpenType features, empowering you to elevate your web typography to international professional standards.

Understanding OpenType Fonts and Their Features

Before we dive into CSS controls, it's essential to understand what OpenType fonts are and why they are so valuable. OpenType, developed jointly by Microsoft and Adobe, is a highly versatile font format that extends the capabilities of older formats like TrueType and PostScript. It's designed to support a vast range of languages and typographic conventions, making it ideal for a global audience.

The true power of OpenType lies in its support for a wide array of typographic features, often referred to as OpenType features or font features. These features allow for advanced stylistic and linguistic adaptations that go beyond simple character substitution. Some of the most common and impactful OpenType features include:

• Ligatures: These are single glyphs that represent two or more characters. Common ligatures include 'fi', 'fl', 'ff', 'ffi', and 'ffl'. They improve readability and aesthetics by combining problematic character pairs that might otherwise overlap or look awkward.
• Contextual Alternates: These features automatically adjust characters based on their surrounding characters, ensuring a more natural flow and consistent appearance, especially in scripts with complex joining rules.
• Swash Variants: These are decorative embellishments that can be added to characters, often for the beginning or ending of words, providing an elegant and expressive touch.
• Stylistic Sets (ss01-ss20): Many OpenType fonts include pre-designed stylistic alternatives for certain characters. These sets allow designers to switch between different designs for letters, numbers, or punctuation, offering a range of aesthetic choices within a single font family.
• Oldstyle Figures (onum): Unlike tabular figures (lining figures), oldstyle figures have ascenders and descenders, resembling lowercase letters. They are particularly suited for body text and historical contexts, blending more harmoniously with the surrounding text.
• Fractions: These are pre-designed typographic fractions that look more refined than stacked diagonal fractions.
• Small Caps: While not strictly an OpenType feature in all cases, fonts often include dedicated small caps glyphs, which are preferable to faux small caps generated by simply scaling uppercase letters.
• Kerning: While kerning is often handled automatically by font metrics, some OpenType features allow for finer control over the spacing between specific character pairs.

These features are typically accessed through desktop publishing software like Adobe InDesign or Illustrator using specific glyph names or feature codes. However, on the web, the primary method for controlling these features is through CSS.

Introducing font-feature-settings

The most fundamental CSS property for controlling OpenType features is font-feature-settings. It allows you to enable or disable specific OpenType features by providing their four-character feature tags. These tags are standardized identifiers defined by the OpenType specification.

Common font-feature-settings Tags

Here are some of the most commonly used feature tags that you can control with font-feature-settings:

• liga: Enables standard ligatures.
• clig: Enables contextual ligatures (often used with `liga`).
• dlig: Enables discretionary ligatures (less common, often for stylistic effect).
• salt: Enables stylistic alternates.
• swsh: Enables swashes.
• onum: Enables oldstyle figures.
• lnum: Enables lining figures (default).
• frac: Enables fractions.
• smcp: Enables small caps.
• cpsp: Enables capitalization spacing.
• kern: Enables kerning (often handled by default).

Using font-feature-settings

The syntax for font-feature-settings is a comma-separated list of feature tags and their desired states:

• 'feature-tag' on: Enables the feature.
• 'feature-tag' off: Disables the feature.
• 'feature-tag' 1: Enables the feature (equivalent to on for many features).
• 'feature-tag' 0: Disables the feature (equivalent to off for many features).
• 'feature-tag' value: For features that support multiple variants (e.g., stylistic sets), a numerical value selects a specific variant.

Example: Enabling Ligatures and Oldstyle Figures

            
body {
  font-family: 'Merriweather', serif;
  font-feature-settings: 'liga' on, 'onum' on;
}

            

              
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In this example, we're applying the 'Merriweather' font to the body. We then enable standard ligatures (`'liga' on`) and oldstyle figures (`'onum' on`). This would mean that character combinations like 'fi' and 'fl' would render as their respective ligature glyphs, and numbers like '123' would use the oldstyle figure designs if the font supports them.

Example: Disabling Ligatures

While ligatures often enhance readability, there might be instances where they are not desired, for example, in code snippets or specific linguistic contexts. You can disable them using:

            
.code-snippet {
  font-family: 'Fira Code', monospace;
  font-feature-settings: 'liga' off;
}

            

              
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Example: Using Stylistic Sets

Many OpenType fonts offer multiple stylistic sets. For instance, a font might have 20 different stylistic sets, allowing for extensive customization. You can access these using tags like ss01 through ss20. The value assigned to the tag determines which stylistic set is used.

            
.artistic-text {
  font-family: 'Playfair Display', serif;
  font-feature-settings: 'ss01' 1; /* Activates the first stylistic set */
}

.alternative-text {
  font-family: 'Playfair Display', serif;
  font-feature-settings: 'ss02' 1; /* Activates the second stylistic set */
}

            

              
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It's crucial to consult the specific documentation for each font to understand which stylistic sets it offers and what stylistic variations they provide. For example, 'Playfair Display' might offer different stylistic alternates for 'q' or 'g' in its stylistic sets.

The `font-variant` Shorthand Property

The font-variant property is a shorthand for several other font-related properties, including some that control OpenType features. While less granular than font-feature-settings for direct OpenType control, it's useful for common stylistic variations:

• font-variant-ligatures: Controls ligatures (e.g., none, normal, contextual, discretionary).
• font-variant-numeric: Controls numerics (e.g., lining-nums, oldstyle-nums, fractions, tabular-nums).
• font-variant-alternates: Controls stylistic alternates (e.g., normal, stylistic(value)).
• font-variant-position: Controls superscripts and subscripts.
• font-variant-caps: Controls capitalization styles (e.g., normal, small-caps, all-small-caps).

Example: Using font-variant-numeric

            
h1 {
  font-family: 'Source Sans Pro', sans-serif;
  font-variant-numeric: oldstyle-nums, fractions;
}

            

              
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This applies oldstyle figures and fractions to the heading if the font supports them. It's a more semantic way to achieve these effects compared to directly using font-feature-settings for these specific features.

The Power of @font-feature-values: Creating Thematic Font Styles

While font-feature-settings is powerful for individual element styling, managing complex typographic rules across a large website can become repetitive and difficult to maintain. This is where the @font-feature-values at-rule shines. It allows you to define custom names for specific OpenType feature settings, making your CSS cleaner, more readable, and easier to manage.

Defining Custom Font Feature Names

The syntax for @font-feature-values involves defining a name for a font family and then specifying custom keywords for OpenType features. This lets you group related feature settings under a single, memorable name.

Example: Defining a 'Classic' Style

Let's say you have a typeface like 'Garamond Premier Pro' which has excellent support for oldstyle figures, ligatures, and stylistic alternates that give it a classic feel. You can define a custom keyword for this style:

            
@font-feature-values 'Garamond Premier Pro' {
  .classic-style {
    ligatures: normal;
    oldstyle-nums: on;
    stylistic-alternates: stylistic(1);
  }

  .modern-style {
    ligatures: none;
    oldstyle-nums: off;
    stylistic-alternates: stylistic(5);
  }
}

            

              
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In this example:

• We've declared a font family name, `'Garamond Premier Pro'`. This name should ideally match the `font-family` name you'll use later.
• We've created a custom keyword, `.classic-style`, and assigned it specific OpenType settings: normal ligatures, oldstyle figures, and the first stylistic alternate.
• We've also defined a `.modern-style` with different settings.

Applying Custom Font Feature Names

Once defined, you can apply these custom keywords using the standard font properties:

            
body {
  font-family: 'Garamond Premier Pro', serif;
}

h1 {
  font-feature-values: 'Garamond Premier Pro' classic-style;
}

p {
  font-feature-values: 'Garamond Premier Pro' modern-style;
}

            

              
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This approach significantly improves the maintainability of your CSS. Instead of repeating complex font-feature-settings declarations, you can use simple, descriptive keywords. This is particularly beneficial when working in international teams or on large projects where consistency is key.

@font-feature-values with Multiple Font Families

You can define these feature value sets for multiple font families within the same stylesheet:

            
@font-feature-values 'Source Serif Pro' {
  .editorial {
    ligatures: normal;
    oldstyle-nums: on;
    swashes: swash(2);
  }
}

@font-feature-values 'Source Sans Pro' {
  .technical {
    ligatures: none;
    tabular-nums: on;
    case: small-caps;
  }
}

            

              
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And then apply them:

            
.article-body {
  font-family: 'Source Serif Pro', serif;
  font-feature-values: 'Source Serif Pro' editorial;
}

.ui-label {
  font-family: 'Source Sans Pro', sans-serif;
  font-feature-values: 'Source Sans Pro' technical;
}

            

              
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Global Typography Considerations

When designing for a global audience, typographic choices have significant implications. OpenType features can be instrumental in adapting your designs to different languages and cultural preferences.

Language-Specific Features

Many OpenType fonts include features that are specifically designed to support particular languages or scripts. For example:

• Contextual Alternates are vital for languages with cursive or joining scripts, like Arabic or Devanagari, ensuring letters connect correctly.
• Language-Specific Ligatures might exist for certain phonetic combinations in various European languages.
• Localized Forms of characters can be included to match specific regional typographic conventions.

The CSS property lang() can be combined with font-feature-settings to apply different typographic styles based on the language of the content.

Example: Language-Specific Styling

Suppose you have a font that supports French typographic conventions, like specific ligatures or punctuation styles, and you want to apply them only to French text.

            
@font-feature-values 'Libre Baskerville' {
  .french-ligatures {
    font-feature-settings: 'flah' on; /* Example: French ligature setting */
  }
}

:lang(fr) p {
  font-family: 'Libre Baskerville', serif;
  font-feature-values: 'Libre Baskerville' french-ligatures;
}

/* For other languages, you might disable or use default */
:lang(en) p {
  font-family: 'Libre Baskerville', serif;
  font-feature-settings: 'flah' off; /* Disable specific French ligature if not applicable */
}

            

              
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Note: The specific feature tags for language-specific features can vary greatly. You'll need to consult the font's documentation for these tags (e.g., `flah` for French ligatures, `rlig` for required ligatures).

Readability Across Devices and Cultures

OpenType features can also significantly impact readability across different devices and for diverse user groups.

• Oldstyle Figures can improve the readability of numerical data in body text, especially for reports or financial information where traditional aesthetics are preferred.
• Fractions make numerical data easier to scan and understand.
• Small Caps are effective for acronyms or initialisms, but overuse can decrease readability, especially in longer passages.

Consider your target audience and the context of the content. For a global audience, prioritizing clarity and legibility might mean opting for simpler, more universally understood typographic settings, or providing options for users to customize their viewing experience.

Font Licensing and Accessibility

When using web fonts, always pay close attention to licensing agreements. Some font licenses may restrict the use of certain OpenType features or require specific attribution. Ensure that your chosen fonts are licensed for web use and for the features you intend to employ.

Furthermore, consider accessibility. While advanced typographic features can enhance aesthetics, ensure they do not hinder readability for users with visual impairments or cognitive differences. Test your designs with accessibility tools and user feedback.

Practical Examples and Best Practices

Let's consolidate our understanding with some practical examples and best practices for implementing OpenType feature control.

1. Enhancing Editorial Content

For articles, blogs, or any long-form text, using OpenType features can create a more refined and readable experience.

            
@font-feature-values 'Merriweather' {
  .editorial {
    ligatures: normal;
    oldstyle-nums: on;
    contextual-alternates: on;
    case: small-caps;
  }
}

.article p {
  font-family: 'Merriweather', serif;
  font-size: 1.1rem;
  line-height: 1.6;
  font-feature-values: 'Merriweather' editorial;
}

.article p .acronym {
  font-feature-values: 'Merriweather' case(small-caps);
}

            

              
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Explanation: This applies ligatures, oldstyle figures, contextual alternates, and small caps for acronyms within paragraphs of an article. The use of `oldstyle-nums` can make numbers in the text blend more naturally.

2. Creating Distinctive Headlines

Headlines are often where you can experiment with more stylistic OpenType features to make them stand out.

            
@font-feature-values 'Playfair Display' {
  .headline-style {
    ligatures: discretionary;
    swashes: swash(3);
    stylistic-alternates: stylistic(2);
  }
}

h1 {
  font-family: 'Playfair Display', serif;
  font-size: 3rem;
  font-weight: 700;
  font-feature-values: 'Playfair Display' headline-style;
}

            

              
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Explanation: This example uses discretionary ligatures, a specific swash variant, and a stylistic alternate to give the main heading a more artistic and unique appearance.

3. Optimizing Data Presentation

For tables, financial reports, or dashboards, tabular figures and precise spacing are crucial.

            
@font-feature-values 'Roboto Mono' {
  .tabular-data {
    tabular-nums: on;
    case: normal;
    ligatures: off;
  }
}

table td {
  font-family: 'Roboto Mono', monospace;
  font-size: 0.9rem;
  font-feature-values: 'Roboto Mono' tabular-data;
}

            

              
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Explanation: This ensures that numbers in tables align perfectly using tabular figures, and disables ligatures that might interfere with numerical readability. Small caps are also turned off to maintain consistent character height.

Best Practices Checklist:

• Know Your Font: Always consult the font's documentation to understand its OpenType feature support and the specific meanings of feature tags and stylistic sets.
• Use @font-feature-values: Leverage this at-rule for cleaner, more maintainable CSS, especially for defining thematic styles.
• Prioritize Readability: While stylistic features are appealing, ensure they don't compromise legibility, especially for body text and global audiences.
• Consider Language: Use CSS lang() to apply language-specific typographic rules where necessary.
• Accessibility First: Test your typographic choices with accessibility in mind. Avoid overly decorative features that might hinder screen readers or users with low vision.
• Performance: Be mindful that enabling many OpenType features can sometimes impact font rendering performance. Test on various devices.
• Browser Support: While modern browsers offer good support for OpenType features via CSS, always check compatibility for older browsers if your audience requires it. font-feature-settings generally has wider support than the more specific font-variant-* properties or @font-feature-values.
• Fallback Fonts: Always define fallback fonts in your CSS to ensure text remains readable even if the primary font fails to load or doesn't support certain features.

Conclusion

CSS font feature values, particularly through the font-feature-settings property and the @font-feature-values at-rule, offer unparalleled control over the sophisticated capabilities of OpenType fonts. By mastering these tools, you can create web experiences that are not only visually stunning but also typographically rich and culturally adaptable.

For a global audience, this level of control is not just about aesthetics; it's about ensuring clarity, legibility, and a connection with diverse linguistic and cultural expectations. Embrace the power of OpenType features to craft web typography that truly speaks to everyone, everywhere.