diff --git a/components/style/properties/longhand/font.mako.rs b/components/style/properties/longhand/font.mako.rs
index a9da305f6a2..230a8839fed 100644
--- a/components/style/properties/longhand/font.mako.rs
+++ b/components/style/properties/longhand/font.mako.rs
@@ -741,6 +741,10 @@ ${helpers.single_keyword_system("font-variant-caps",
         }
     % endif
 
+    /// This is the ratio applied for font-size: larger
+    /// and smaller by both Firefox and Chrome
+    const LARGER_FONT_SIZE_RATIO: f32 = 1.2;
+
     impl SpecifiedValue {
         /// https://html.spec.whatwg.org/multipage/#rules-for-parsing-a-legacy-font-size
         pub fn from_html_size(size: u8) -> Self {
@@ -768,6 +772,10 @@ ${helpers.single_keyword_system("font-variant-caps",
                         return Some(em)
                     }
                 }
+            } else if let SpecifiedValue::Larger = *self {
+                return Some(LARGER_FONT_SIZE_RATIO)
+            } else if let SpecifiedValue::Smaller = *self {
+                return Some(1. / LARGER_FONT_SIZE_RATIO)
             }
             None
         }
@@ -799,11 +807,11 @@ ${helpers.single_keyword_system("font-variant-caps",
                     key.to_computed_value(context).scale_by(fraction)
                 }
                 SpecifiedValue::Smaller => {
-                    FontRelativeLength::Em(0.85)
+                    FontRelativeLength::Em(1. / LARGER_FONT_SIZE_RATIO)
                         .to_computed_value(context, base_size)
                 }
                 SpecifiedValue::Larger => {
-                    FontRelativeLength::Em(1.2)
+                    FontRelativeLength::Em(LARGER_FONT_SIZE_RATIO)
                         .to_computed_value(context, base_size)
                 }
 
diff --git a/tests/wpt/mozilla/meta/MANIFEST.json b/tests/wpt/mozilla/meta/MANIFEST.json
index 6d77e39770b..1b33db843e9 100644
--- a/tests/wpt/mozilla/meta/MANIFEST.json
+++ b/tests/wpt/mozilla/meta/MANIFEST.json
@@ -21375,11 +21375,11 @@
    "support"
   ],
   "css/font_size.html": [
-   "1647905b6a68c9bb577afcbcca203208de524a88",
+   "7c944acb4f7d909083888a355e6d664c23081b99",
    "reftest"
   ],
   "css/font_size_ref.html": [
-   "b0b486eb2cbc8537edae3a7c76b08c8ecc9a0ef3",
+   "cc34cbb1aae8e0d3738476bb09264f0fade78ba6",
    "support"
   ],
   "css/font_style.html": [
diff --git a/tests/wpt/mozilla/tests/css/font_size.html b/tests/wpt/mozilla/tests/css/font_size.html
index 921e2887d2b..4ffd55261f2 100644
--- a/tests/wpt/mozilla/tests/css/font_size.html
+++ b/tests/wpt/mozilla/tests/css/font_size.html
@@ -8,7 +8,7 @@
 <p style="font-size: 2em">2em is 40px.
 <p style="font-size: 2rem">2rem is 14px.
 <p style="font-size: 200%">200% is 40px.
-<p style="font-size: smaller">smaller is 17px.
+<p style="font-size: smaller">smaller is 16.66666…px.
 <p style="font-size: larger">larger is 24px.
 <p style="font-size: xx-small">xx-small is 9.6px.
 <p style="font-size: x-small">x-small is 12px.
diff --git a/tests/wpt/mozilla/tests/css/font_size_ref.html b/tests/wpt/mozilla/tests/css/font_size_ref.html
index a046358a884..6bcf5433885 100644
--- a/tests/wpt/mozilla/tests/css/font_size_ref.html
+++ b/tests/wpt/mozilla/tests/css/font_size_ref.html
@@ -6,7 +6,7 @@
 <p style="font-size: 40px">2em is 40px.
 <p style="font-size: 14px">2rem is 14px.
 <p style="font-size: 40px">200% is 40px.
-<p style="font-size: 17px">smaller is 17px.
+<p style="font-size: 16.6666666666666666666666666666px">smaller is 16.66666…px.
 <p style="font-size: 24px">larger is 24px.
 <p style="font-size: 9.6px">xx-small is 9.6px.
 <p style="font-size: 12px">x-small is 12px.