diff --git a/after/syntax/css/css-flexbox-1.vim b/after/syntax/css/css-flexbox-1.vim
index 273c860..3049b64 100644
--- a/after/syntax/css/css-flexbox-1.vim
+++ b/after/syntax/css/css-flexbox-1.vim
@@ -4,3 +4,4 @@ syn match cssFontProp contained "\<flex\(-\(basis\|direction\|flow\|grow\|shrink
 syn keyword cssFontAttr contained flex row wrap
 syn match cssFontAttr contained "\<inline-flex\>"
 syn match cssFontAttr contained "\<\(row\|column\|wrap\)-reverse\>"
+syn match cssFontAttr contained "\<main-size\>"
diff --git a/test/test.css b/test/test.css
index 8985ea7..9f4ba8f 100644
--- a/test/test.css
+++ b/test/test.css
@@ -144,7 +144,7 @@
   flex: column-reverse;
   flex-grow: wrap;
   flex-shrink: wrap-reverse;
-  flex-basis: auto;
+  flex-basis: main-size;
 }
 
 .fonts {
diff --git a/test/test.html b/test/test.html
index 7110036..09b8ab2 100644
--- a/test/test.html
+++ b/test/test.html
@@ -154,7 +154,7 @@
         flex: column-reverse;
         flex-grow: wrap;
         flex-shrink: wrap-reverse;
-        flex-basis: auto;
+        flex-basis: main-size;
       }
 
       .fonts {