Font Validation: Testing Quality & Integrity

OpenType Sanitizer (OTS)

OTS is Google's security-focused font validator. Chrome, Firefox, and other browsers use OTS to validate fonts before rendering. If OTS rejects your font, it won't display in browsers.

# Install OTS
# macOS
brew install ots

# Linux
sudo apt-get install opentype-sanitizer

# Run validation
ots-sanitize myfont.woff2

# Output examples:
# Success: "File sanitized successfully!"
# Failure: "ERROR: Bad checksum for head table"
# Failure: "ERROR: Table is too short"

OTS was designed as a browser security sandbox because malformed fonts could trigger memory corruption vulnerabilities in OS-level font rasterizers. Windows GDI and macOS Core Text both accumulated CVEs related to font parsing in the late 2000s, allowing attackers to execute arbitrary code through carefully crafted web fonts. OTS prevents this by reconstructing a sanitized font from scratch rather than passing raw bytes to the OS parser—every table is independently parsed, validated against the spec, and re-serialized into a clean output. This conservative approach means even structurally unusual but technically valid fonts may be rejected if they trigger OTS's strict parsing assumptions.

OTS failures are binary: pass or fail, with no warnings or partial outcomes. When a font fails OTS, the browser silently falls back to the next font in the CSS font stack, with no visible error message for end users. The problem only reveals itself to developers who check the browser console, notice unexpected fallback font rendering, or examine network requests showing a 200 response that produces no font. This silent failure mode is why pre-deployment OTS testing is non-negotiable. The command-line ots-sanitize tool replicates the browser check locally and can be integrated into build pipelines as a required gate before any font deployment.

Font Bakery

Font Bakery is an open-source Python tool that runs 200+ quality checks. It's required for Google Fonts submissions and recommended for all professional font work.

# Install Font Bakery
pip install fontbakery

# Run universal checks (all fonts)
fontbakery check-universal myfont.ttf

# Run Google Fonts profile (stricter)
fontbakery check-googlefonts myfont.ttf

# Run specific checks
fontbakery check-outline myfont.ttf
fontbakery check-opentype myfont.ttf

# Output to HTML report
fontbakery check-universal myfont.ttf --html report.html

$ fontbakery check-googlefonts MyFont-Regular.ttf

>> com.google.fonts/check/name/family_name_compliance
   [FAIL] Name ID 1 should not contain "Regular" substring.
   MyFont Regular → should be: MyFont

>> com.google.fonts/check/metrics_winascent_and_windesced
   [WARN] OS/2 usWinAscent value 1200 is too large.
   Recommended: match your tallest glyph height (900)

>> com.google.fonts/check/glyf_non_symmetric_glyphs
   [PASS] All glyphs have symmetric y-coordinates.

>> com.google.fonts/check/ligature_carets
   [INFO] Font has ligatures but no caret positions defined.
   This is optional but recommended for cursor positioning.

Results: 1 FAIL, 1 WARN, 1 INFO, 1 PASS

Font Bakery organizes its checks into profiles. The universal profile covers checks applicable to any font regardless of distribution channel: OpenType specification compliance, name table correctness, outline quality, and metric consistency. The Google Fonts profile adds requirements specific to the library, including copyright string formatting, required name IDs, minimum Unicode character coverage across the Basic Latin and Latin-1 Supplement blocks, and vertical metrics standards that ensure consistent line spacing in the Google Fonts serving infrastructure. Foundries distributing through other channels typically run only the universal profile to avoid false positives from Google Fonts's opinionated conventions that do not apply universally.

Font Bakery's extensible architecture supports custom check plugins written as Python modules, allowing foundries to encode house-style rules and distribution requirements as executable validation logic alongside the standard checks. Check IDs follow a hierarchical naming scheme: com.google.fonts/check/metrics_winascent_and_windesced identifies namespace, category, and subject. Results are exported as structured JSON, enabling integration with monitoring dashboards and trend tracking across font releases. A mature font development pipeline will accumulate a custom check profile over time that catches the specific classes of errors that historically caused problems for that foundry's fonts on their target platforms.

Common Validation Errors

Related Technical Guides