Add rationale for OS/2.fsSelection check

com.google.fonts/check/fsselection
On the Open Type Profile.

(PR #4570)

CHANGELOG.md

@@ -29,6 +29,7 @@ A more detailed list of changes is available in the corresponding milestones for
   - **[com.google.fonts/check/family/panose_familytype]:** Add rationale and improve message when there are inconsistencies. (PR #4570)
   - **[com.google.fonts/check/xavgcharwidth]:** Added rationale. Also, when glyphs needed to compute xAvgCharWidth are missing, it is considered critical, so it is now results in a FATAL-level result. (PR #4570)
   - **[com.adobe.fonts/check/fsselection_matches_macstyle]:** Improve log messages to include the bit values. (PR #4570)
+  - **[com.google.fonts/check/fsselection]:** Added rationale. (PR #4570)
 
 
 ## 0.12.0a2 (2024-Feb-21)

Lib/fontbakery/checks/opentype/os2.py

@@ -372,6 +372,16 @@ def com_thetypefounders_check_vendor_id(config, ttFont):
     id="com.google.fonts/check/fsselection",
     conditions=["style"],
     proposal="legacy:check/129",
+    rationale="""
+    The OS/2.fsSelection field is a bit field used to specify the stylistic
+    qualities of the font - in particular, it specifies to some operating
+    systems whether the font is italic (bit 0), bold (bit 5) or regular
+    (bit 6).
+
+    This check verifies that the fsSelection field is set correctly for the
+    font style. For a family of static fonts created in GlyphsApp, this is
+    set by using the style linking checkboxes in the exports settings.
+    """,
 )
 def com_google_fonts_check_fsselection(ttFont, style):
     """Checking OS/2 fsSelection value."""