Font Loading Strategy Checker

The CSS font-display descriptor, defined in the CSS Fonts Module Level 4 specification, controls how a web font is rendered while it is loading. It determines the block period (how long the browser hides text while waiting for the font), the swap period (how long the browser shows a fallback font before swapping), and the failure period (what happens if the font never loads). Choosing the wrong font-display value directly impacts Cumulative Layout Shift (CLS) and Largest Contentful Paint (LCP)- two of Google's Core Web Vitals metrics. Our font loading strategy checker helps you pick the optimal value for every font on your page.

1. Enter Your Font Details: Provide the font family name, estimated file size (KB), and estimated load time (ms). Choose from quick presets for common scenarios like body text (30 KB), headings (50 KB), or large CJK fonts (500 KB). The font loading strategy checker uses these inputs to model realistic loading behaviour.
2. Compare All Strategies Side-by-Side: The tool analyses all five CSS font-display values - auto, block, swap, fallback, and optional- and presents a comparison grid showing timing metrics, CLS ratings, and visual timelines for each. A recommended strategy is highlighted based on your font's size and load profile.
3. Review and Copy the Recommended CSS: Click any strategy card for a detailed view with an interactive timeline of block, fallback, and swap phases. The generated @font-face CSS with your chosen font-display value is ready to copy and paste into your stylesheets.

• Block:The browser hides text for up to 3 seconds while the font loads (FOIT - Flash of Invisible Text). If the font loads within 3 seconds, text appears in the correct typeface with no layout shift. If the font doesn't load, text appears in a fallback font. Best for hero headings where visual consistency matters more than fast rendering.
• Swap: Text renders immediately in a fallback font, then swaps to the custom font when it loads (FOUT - Flash of Unstyled Text). This gives the best First Paint time but causes layout shift when the fonts swap. Best for brand-critical text where showing the correct typeface is essential.
• Fallback: A compromise - a short block period (~100ms) allows cached fonts to load without swap flash, then falls back to the system font. The font only swaps if it loads within ~3 seconds. Best for body text where you want fast rendering with occasional custom typography.
• Optional: A very short block period (~100ms), then the font is considered optional. If it loads in time (e.g., from cache), it renders; otherwise the fallback persists. The font never swaps in on slow connections. Best for decorative and non-critical fonts.
• Auto: The browser chooses the strategy. In most modern browsers, this defaults to behaviour similar to "block". Avoidauto when you need predictable font rendering behaviour.

When a font swap occurs, the layout can shift because the metrics of the custom font (cap height, x-height, ascender/descender ratios, advance widths, and glyph bounding boxes) differ from the fallback font. The CLS contribution of a font swap depends on: the metrics difference between the custom and fallback font (typically larger for display/heading fonts and smaller for body fonts), the text length affected (headings shift more than single words), and the number of elements using the font. Preloading the font with <link rel="preload">, using font-display: optional, or size-adjust and ascent-override in the @font-facedeclaration can mitigate these shifts. Our font loading strategy checker estimates the CLS contribution for each strategy based on your font's size and load characteristics.