--- a/build/docs/mozinfo.rst
+++ b/build/docs/mozinfo.rst
@@ -139,16 +139,23 @@ release_or_beta
 sm_promise
    Whether spidermonkey promises have been enabled or not.  This is set
    by adding --enable-sm-promise to the mozconfig file.
 
    Values are ``true`` and ``false``.
 
    Always defined.
 
+stylo
+   Whether the Stylo styling system is being used.
+
+   Values are ``true`` and ``false``.
+
+   Always defined.
+
 tests_enabled
    Whether tests are enabled for this build.
 
    Values are ``true`` and ``false``.
 
    Always defined.
 
 toolkit

--- a/python/mozbuild/mozbuild/mozinfo.py
+++ b/python/mozbuild/mozbuild/mozinfo.py
@@ -81,16 +81,17 @@ def build_dict(config, env=os.environ):
     d['debug'] = substs.get('MOZ_DEBUG') == '1'
     d['nightly_build'] = substs.get('NIGHTLY_BUILD') == '1'
     d['release_or_beta'] = substs.get('RELEASE_OR_BETA') == '1'
     d['pgo'] = substs.get('MOZ_PGO') == '1'
     d['crashreporter'] = bool(substs.get('MOZ_CRASHREPORTER'))
     d['datareporting'] = bool(substs.get('MOZ_DATA_REPORTING'))
     d['healthreport'] = substs.get('MOZ_SERVICES_HEALTHREPORT') == '1'
     d['sync'] = substs.get('MOZ_SERVICES_SYNC') == '1'
+    d['stylo'] = substs.get('MOZ_STYLO') == '1'
     d['asan'] = substs.get('MOZ_ASAN') == '1'
     d['tsan'] = substs.get('MOZ_TSAN') == '1'
     d['telemetry'] = substs.get('MOZ_TELEMETRY_REPORTING') == '1'
     d['tests_enabled'] = substs.get('ENABLE_TESTS') == "1"
     d['bin_suffix'] = substs.get('BIN_SUFFIX', '')
     d['addon_signing'] = substs.get('MOZ_ADDON_SIGNING') == '1'
     d['require_signing'] = substs.get('MOZ_REQUIRE_SIGNING') == '1'
     d['official'] = bool(substs.get('MOZILLA_OFFICIAL'))