diff --git a/css/syntaxes.json b/css/syntaxes.json
index 8d075acb..8598ee36 100644
--- a/css/syntaxes.json
+++ b/css/syntaxes.json
@@ -17,6 +17,9 @@
   "anchor-name": {
     "syntax": "<dashed-ident>"
   },
+  "anchor-size": {
+    "syntax": "width | height | block | inline | self-block | self-inline"
+  },
   "anchor-size()": {
     "syntax": "anchor-size( [ <anchor-name> || <anchor-size> ]? , <length-percentage>? )"
   },