diff --git a/components/style/values/specified/gecko.rs b/components/style/values/specified/gecko.rs
index b36cd582aed0..486a33eaf1ea 100644
--- a/components/style/values/specified/gecko.rs
+++ b/components/style/values/specified/gecko.rs
@@ -81,6 +81,11 @@ impl ToNsCssValue for PixelOrPercentage {
 }
 
 /// The value of an IntersectionObserver's rootMargin property.
+///
+/// Only bare px or percentage values are allowed. Other length units and
+/// calc() values are not allowed.
+///
+/// https://w3c.github.io/IntersectionObserver/#parse-a-root-margin
 pub struct IntersectionObserverRootMargin(pub Rect<PixelOrPercentage>);
 
 impl Parse for IntersectionObserverRootMargin {
diff --git a/tests/wpt/metadata/MANIFEST.json b/tests/wpt/metadata/MANIFEST.json
index b06f515f028d..7d35c4584904 100644
--- a/tests/wpt/metadata/MANIFEST.json
+++ b/tests/wpt/metadata/MANIFEST.json
@@ -557079,7 +557079,7 @@
    "testharness"
   ],
   "intersection-observer/observer-exceptions.html": [
-   "28ccc6905713894b43033e30949170439215bf2e",
+   "85d5416475091c1020e0ff92b15f841bf58c606e",
    "testharness"
   ],
   "intersection-observer/observer-in-iframe.html": [
diff --git a/tests/wpt/web-platform-tests/intersection-observer/observer-exceptions.html b/tests/wpt/web-platform-tests/intersection-observer/observer-exceptions.html
index b44b7bd0cc13..d4f178be51c3 100644
--- a/tests/wpt/web-platform-tests/intersection-observer/observer-exceptions.html
+++ b/tests/wpt/web-platform-tests/intersection-observer/observer-exceptions.html
@@ -31,7 +31,19 @@
   assert_throws("SYNTAX_ERR", function() {
     new IntersectionObserver(e => {}, {rootMargin: "auto"})
   })
-}, 'IntersectionObserver constructor width { rootMargin: "auto" }');
+}, 'IntersectionObserver constructor with { rootMargin: "auto" }');
+
+test(function () {
+  assert_throws("SYNTAX_ERR", function() {
+    new IntersectionObserver(e => {}, {rootMargin: "calc(1px + 2px)"})
+  })
+}, 'IntersectionObserver constructor with { rootMargin: "calc(1px + 2px)" }');
+
+test(function () {
+  assert_throws("SYNTAX_ERR", function() {
+    new IntersectionObserver(e => {}, {rootMargin: "1px !important"})
+  })
+}, 'IntersectionObserver constructor with { rootMargin: "1px !important" }');
 
 test(function () {
   assert_throws("SYNTAX_ERR", function() {