--- a/layout/reftests/w3c-css/received/reftest.list
+++ b/layout/reftests/w3c-css/received/reftest.list
@@ -235,17 +235,17 @@ fuzzy-if(OSX,40,6) == css-values-3/ch-un
 == css-values-3/initial-background-color.html css-values-3/reference/all-green.html
 == css-values-3/vh-calc-support-pct.html css-values-3/reference/all-green.html
 == css-values-3/vh-calc-support.html css-values-3/reference/all-green.html
 == css-values-3/vh-em-inherit.html css-values-3/reference/all-green.html
 == css-values-3/vh-inherit.html css-values-3/reference/all-green.html
 == css-values-3/vh-interpolate-pct.html css-values-3/reference/all-green.html
 == css-values-3/vh-interpolate-px.html css-values-3/reference/all-green.html
 == css-values-3/vh-interpolate-vh.html css-values-3/reference/all-green.html
-fails-if(styloVsGecko||stylo) == css-values-3/vh-support-atviewport.html css-values-3/reference/all-green.html
+== css-values-3/vh-support-atviewport.html css-values-3/reference/all-green.html
 == css-values-3/vh-support-margin.html css-values-3/reference/all-green.html
 skip == css-values-3/vh-support-transform-origin.html css-values-3/reference/all-green.html
 skip == css-values-3/vh-support-transform-translate.html css-values-3/reference/all-green.html
 == css-values-3/vh-support.html css-values-3/reference/all-green.html
 == css-values-3/vh-zero-support.html css-values-3/reference/all-green.html
 skip == css-values-3/vh_not_refreshing_on_chrome.html css-values-3/reference/vh_not_refreshing_on_chrome-ref.html
 skip == css-values-3/vh_not_refreshing_on_chrome_iframe.html css-values-3/reference/vh_not_refreshing_on_chrome-ref.html
 == css-writing-modes-3/abs-pos-non-replaced-icb-vlr-003.xht css21/reference/ref-filled-green-100px-square.xht

--- a/servo/components/style/stylesheets.rs
+++ b/servo/components/style/stylesheets.rs
@@ -1699,17 +1699,17 @@ impl<'a, 'b> NestedRuleParser<'a, 'b> {
 
 #[cfg(feature = "servo")]
 fn is_viewport_enabled() -> bool {
     PREFS.get("layout.viewport.enabled").as_boolean().unwrap_or(false)
 }
 
 #[cfg(not(feature = "servo"))]
 fn is_viewport_enabled() -> bool {
-    true
+    false // Gecko doesn't support @viewport
 }
 
 impl<'a, 'b> AtRuleParser for NestedRuleParser<'a, 'b> {
     type Prelude = AtRulePrelude;
     type AtRule = CssRule;
 
     fn parse_prelude(&mut self, name: &str, input: &mut Parser)
                      -> Result<AtRuleType<AtRulePrelude, CssRule>, ()> {