How to Inspect CSS font-feature-settings JIS04 Alternate Full-Width Forms in Safari Web Inspector

Precision East Asian typography on the web demands strict alignment between Latin numerals, punctuation, and Japanese CJK glyphs. When rendering tabular reports, technical documentation, or formal legal notices in Japanese, setting OpenType jp04 (JIS2004 standard forms) alongside falt (Alternate Full-Width Forms) ensures that proportional characters expand into balanced, full-width zenkaku dimensions. Apple Safari’s Web Inspector provides native font analysis tools that simplify testing and validation of these advanced layout features.

Understanding the Synergy Between jp04 and Alternate Full-Width (falt) Glyphs

The jp04 OpenType feature updates historical character representations to conform with contemporary Japanese publishing standards defined in JIS X 0213:2004. Concurrently, the falt tag replaces specific proportional glyphs with alternate full-width variants without modifying the underlying character encoding. This ensures pristine vertical alignment in multi-column tables and vertical writing layouts (writing-mode: vertical-rl).

Declare these OpenType properties using modern high-level CSS properties with fallbacks:

.cjk-formal-document {
  font-family: "Hiragino Kaku Gothic ProN", "Yu Gothic", sans-serif;
  font-variant-east-asian: jis04;
  font-feature-settings: "jp04" 1, "falt" 1;
  text-spacing-trim: space-first;
  line-break: strict;
}

Validating Alternate Full-Width Forms in Safari Web Inspector

To inspect and audit OpenType glyph substitution inside Apple Safari:

1. Launch Safari and navigate to your target page.
2. Right-click on the Japanese text passage and click Inspect Element.
3. Navigate to the Elements tab and select the Font sub-panel located in the right-hand inspection drawer.
4. Expand the Font Features disclosure widget.
5. Verify that both JIS2004 Forms (jp04) and Alternate Full-Width (falt) are enabled and highlighted in blue.
6. Use the live text editing input within the Font panel to confirm that alphanumeric inputs immediately expand into zenkaku em-square dimensions.

Troubleshooting Glyphs and Fallback Rendering

When font features fail to apply as anticipated in Safari, verify the following configuration vectors:

• Subset Stripping: Ensure your web font distribution pipeline has not stripped the falt feature table during WOFF2 subsetting.
• CSS Shorthand Precedence: Be cautious when using the font: shorthand property on child selectors, as it resets font-variant and font-feature-settings to initial values.
• Text Orientation: When pairing full-width glyphs with vertical layouts, explicitly set text-orientation: upright to keep Roman characters upright alongside kanji.