diff --git a/css/syntaxes.json b/css/syntaxes.json
index 8598ee36..d9f4ab09 100644
--- a/css/syntaxes.json
+++ b/css/syntaxes.json
@@ -17,6 +17,9 @@
   "anchor-name": {
     "syntax": "<dashed-ident>"
   },
+  "anchor-side": {
+    "syntax": "inside | outside | top | left | right | bottom | start | end | self-start | self-end | <percentage> | center"
+  },
   "anchor-size": {
     "syntax": "width | height | block | inline | self-block | self-inline"
   },