Fine-Tuning Font Alignment: When to Use (and Avoid) @font-face.ascent-override

What it is

• It allows you to override the default ascent metric of a font that you're loading using @font-face.
• @font-face.ascent-override is a descriptor within the @font-face rule in CSS.

What the ascent metric is

• The ascent metric is a typographic term that refers to the distance, in pixels or other units, from the font's baseline (where most characters sit) to the topmost point of non-ascending characters (typically uppercase letters and some punctuation).

Why you might use it

• You might use ascent-override in a few scenarios:
  • Aligning fallback fonts
    When using a custom font as your primary choice and a fallback font (like a system font) as a backup, the ascent metrics might differ between the two fonts. This can cause layout issues where lines of text appear misaligned. ascent-override lets you adjust the fallback font's ascent to match the primary font, ensuring consistent line heights.
  • Fine-tuning layout
    In rare cases, you might use ascent-override for more precise layout control, but this is generally not recommended as it can lead to unexpected behavior if not used carefully.

How to use it

1. Define the @font-face rule for your custom font.
2. Within the @font-face rule, include the ascent-override descriptor followed by a value:
   • The value can be a specific pixel value (e.g., ascent-override: 12px;) to set a fixed ascent height.
   • You can also use normal (the default behavior) or a percentage (relative to the font's em size) for more flexibility.

Example

@font-face {
  font-family: "MyCustomFont";
  src: url("myfont.woff2") format("woff2");
  /* ... other font-face properties */
  ascent-override: 14px; /* Adjust the ascent to 14 pixels */
}

.my-text {
  font-family: "MyCustomFont", fallback-font; /* Fallback font is used if "MyCustomFont" fails to load */
}

• Modern browsers have improved font loading and fallback mechanisms, so the need for ascent-override is less common than in the past.
• Consider using CSS properties like line-height for more general control over line spacing.
• Use ascent-override sparingly as it can affect layout in unexpected ways, especially if you're not careful about the value you choose.

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Overriding ascent metric of a fallback font

This example shows how to adjust the ascent metric of a fallback font (Arial Bold) to match the ascent of a custom font ("MyCustomFont"). This ensures consistent line heights:

@font-face {
  font-family: "MyCustomFont";
  src: url("myfont.woff2") format("woff2");
}

/* Fallback font (Arial Bold) with adjusted ascent */
@font-face {
  font-family: "Arial Bold";
  src: local(Arial Bold);
  ascent-override: 14px; /* Adjust ascent to match "MyCustomFont" (assumed to be 14px) */
}

.my-text {
  font-family: "MyCustomFont", "Arial Bold"; /* "Arial Bold" is used as fallback */
}

Overriding with a percentage value

This example uses a percentage value for ascent-override to adjust the ascent of a fallback font relative to its em size:

@font-face {
  font-family: "MyCustomFont";
  src: url("myfont.woff2") format("woff2");
}

/* Fallback font with ascent adjusted to 110% of its em size */
@font-face {
  font-family: "FallbackFont";
  src: url("fallbackfont.woff2") format("woff2");
  ascent-override: 110%;
}

.my-text {
  font-family: "MyCustomFont", "FallbackFont";
}

Remember to replace "MyCustomFont", "myfont.woff2", "FallbackFont", and "fallbackfont.woff2" with your actual font names and file paths.

• Consider testing your CSS across different browsers to ensure consistent rendering, as browser support for ascent-override might vary slightly.
• In both examples, make sure the assumed ascent value (14px in the first example) for the custom font is accurate for proper alignment.

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line-height property

The line-height property is the more common and recommended approach for controlling line spacing and alignment. It sets the minimum height for a line box, which includes the ascent, descent, and line gap of the font. It can be specified in various units:

.my-text {
  font-family: "MyCustomFont", fallback-font;
  line-height: 1.2; /* Sets line height to 1.2 times the font size */
}

System fonts

If consistent line heights are your primary concern, consider using system fonts (like Arial or Times New Roman) as your fallback fonts. These fonts are generally well-aligned and consistent across different browsers.

.my-text {
  font-family: "MyCustomFont", Arial, sans-serif;
}

Font loading strategies

• Font fallback with similar ascent
  Choose fallback fonts that have a known ascent metric similar to your custom font. This can help maintain layout consistency even without ascent-override.
• Font display
  Use the font-display property within @font-face to control how the font is displayed during loading. Options like swap can help minimize layout shifts caused by font loading.

When to avoid ascent-override

• Modern browsers have improved font loading and fallback mechanisms, reducing the need for ascent-override in many cases.
• Overriding the font's inherent metrics might lead to unintended visual effects.
• It can be difficult to ensure accurate and consistent results across different browsers.

• You understand the potential drawbacks and can test thoroughly across browsers.
• You've exhausted other options and need very specific control over line spacing.