Microsoft PowerPoint: Incomplete OpenType GPOS kerning support and unreliable fallback to legacy kern subtables

PowerPoint for Mac does not provide the same level of OpenType layout support as advanced text engines used in Adobe apps, browsers, or some macOS-native apps. The available documentation and guidance focus on the font-side requirements rather than guaranteeing full GPOS feature parity across all Office apps.

From the OpenType specification and recommendations:

• Modern kerning is expected to be implemented in the GPOS table using the kern feature, especially for variable fonts and for fonts with CFF outlines.
• The legacy kern table is retained primarily for compatibility with older or simpler layout engines and is significantly more limited than GPOS (no variations, limited formats, and platform-specific behavior).
• For cross‑platform and Windows compatibility, fonts should:
  • Use GPOS for primary kerning.
  • If a legacy kern table is present, restrict it to a single format‑0 subtable; Windows and many layout engines will ignore additional subtables and unsupported formats.

Key points from the OpenType guidance relevant to the scenario:

1. GPOS vs kern precedence
   When both GPOS and kern are present, OpenType layout engines are expected to:
   • Use GPOS kern lookups if any exist for the resolved script/language system.
   • Fall back to the legacy kern table only if there are zero kern lookups in GPOS for that language system.
2. Limitations of the legacy kern table
   • Only format‑0 pair‑kerning subtables are broadly supported on Windows.
   • Multiple subtables and Apple-specific extensions are not supported on Windows and should not be relied on for cross‑platform behavior.
   • OpenType Font Variations do not support variation data in kern; variable fonts must implement kerning via GPOS.
3. CFF-based OpenType fonts
   • For OpenType fonts with CFF outlines, Windows GDI’s CFF driver ignores the legacy kern table when preparing kerning pairs; kerning must be in GPOS.

Given these constraints, the behavior observed in PowerPoint for Mac is consistent with the general guidance that:

• The legacy kern table is effectively a compatibility path and not guaranteed to behave like a full OpenType layout engine.
• Only the first format‑0 kern subtable should be assumed to work reliably across Microsoft environments; additional subtables or formats may be ignored.
• Full, class-based, script‑aware kerning is expected to be implemented via GPOS, but Office apps may not expose or honor all GPOS features in the same way as specialized layout engines.

For font development targeting Office (including PowerPoint), the documented best practices are:

• Implement complete kerning in GPOS using the kern feature.
• If compatibility with legacy or limited engines (including some Office scenarios) is required, add a single, flattened format‑0 kern subtable containing essential pairs only, keeping in mind that:
  • Only that first subtable is reliably used.
  • Kerning variations and advanced class structures must remain in GPOS.

There is no documented guarantee that PowerPoint will fully match Adobe or browser engines in GPOS support, nor that it will honor multiple kern subtables. The current OpenType recommendations instead guide font authors to:

• Prefer GPOS for modern kerning.
• Use a minimal, spec‑compliant, single‑subtable legacy kern only as a fallback for environments with incomplete GPOS support, which includes some Office scenarios.

References:

• Recommendations for OpenType Fonts (OpenType 1.8.3)
• Recommendations for OpenType Fonts (OpenType 1.8.1)
• kern - Kerning (OpenType 1.6)
• kern - Kerning (OpenType 1.8.2)