diff --git a/components/style/properties/data.py b/components/style/properties/data.py
index c0510d0c13f..32ab3079f03 100644
--- a/components/style/properties/data.py
+++ b/components/style/properties/data.py
@@ -151,7 +151,7 @@ class Longhand(object):
                  allowed_in_keyframe_block=True, cast_type='u8',
                  has_uncacheable_values=False, logical=False, alias=None, extra_prefixes=None, boxed=False,
                  flags=None, allowed_in_page_rule=False, allow_quirks=False, ignored_when_colors_disabled=False,
-                 vector=False):
+                 gecko_pref_ident=None, vector=False):
         self.name = name
         if not spec:
             raise TypeError("Spec should be specified for %s" % name)
@@ -178,6 +178,7 @@ class Longhand(object):
         self.allowed_in_page_rule = arg_to_bool(allowed_in_page_rule)
         self.allow_quirks = allow_quirks
         self.ignored_when_colors_disabled = ignored_when_colors_disabled
+        self.gecko_pref_ident = gecko_pref_ident or self.ident
         self.is_vector = vector
 
         # https://drafts.csswg.org/css-animations/#keyframes
@@ -214,7 +215,7 @@ class Longhand(object):
 class Shorthand(object):
     def __init__(self, name, sub_properties, spec=None, experimental=False, internal=False,
                  allowed_in_keyframe_block=True, alias=None, extra_prefixes=None,
-                 allowed_in_page_rule=False, flags=None):
+                 allowed_in_page_rule=False, flags=None, gecko_pref_ident=None):
         self.name = name
         if not spec:
             raise TypeError("Spec should be specified for %s" % name)
@@ -229,6 +230,7 @@ class Shorthand(object):
         self.extra_prefixes = extra_prefixes.split() if extra_prefixes else []
         self.allowed_in_page_rule = arg_to_bool(allowed_in_page_rule)
         self.flags = flags.split() if flags else []
+        self.gecko_pref_ident = gecko_pref_ident or self.ident
 
         # https://drafts.csswg.org/css-animations/#keyframes
         # > The <declaration-list> inside of <keyframe-block> accepts any CSS property
diff --git a/components/style/properties/longhand/box.mako.rs b/components/style/properties/longhand/box.mako.rs
index 871eb37513d..82cd921709a 100644
--- a/components/style/properties/longhand/box.mako.rs
+++ b/components/style/properties/longhand/box.mako.rs
@@ -171,6 +171,7 @@ ${helpers.single_keyword("position", "static absolute relative fixed",
                                   gecko_enum_prefix="StyleFloat"
                                   gecko_inexhaustive="True"
                                   gecko_ffi_name="mFloat"
+                                  gecko_pref_ident="float_"
                                   spec="https://drafts.csswg.org/css-box/#propdef-float">
     no_viewport_percentage!(SpecifiedValue);
     impl ToComputedValue for SpecifiedValue {
diff --git a/components/style/properties/properties.mako.rs b/components/style/properties/properties.mako.rs
index 19d9cc54b0e..cb86628310d 100644
--- a/components/style/properties/properties.mako.rs
+++ b/components/style/properties/properties.mako.rs
@@ -1179,30 +1179,46 @@ impl ToCss for PropertyDeclaration {
     }
 }
 
-<%def name="property_pref_check(property)">
-    % if property.experimental and product == "servo":
-        if !PREFS.get("${property.experimental}")
-            .as_boolean().unwrap_or(false) {
-            return Err(PropertyDeclarationParseError::ExperimentalProperty)
-        }
-    % endif
-    % if product == "gecko":
-        <%
-            # gecko can't use the identifier `float`
-            # and instead uses `float_`
-            # XXXManishearth make this an attr on the property
-            # itself?
-            pref_ident = property.ident
-            if pref_ident == "float":
-                pref_ident = "float_"
-        %>
-        if structs::root::mozilla::SERVO_PREF_ENABLED_${pref_ident} {
-            let id = structs::${helpers.to_nscsspropertyid(property.ident)};
-            let enabled = unsafe { bindings::Gecko_PropertyId_IsPrefEnabled(id) };
-            if !enabled {
-                return Err(PropertyDeclarationParseError::ExperimentalProperty)
+<%def name="property_exposure_check(property)">
+    // For properties that are experimental but not internal, the pref will
+    // control its availability in all sheets.   For properties that are
+    // both experimental and internal, the pref only controls its
+    // availability in non-UA sheets (and in UA sheets it is always available).
+    let is_experimental =
+        % if property.experimental and product == "servo":
+            true;
+        % elif product == "gecko":
+            structs::root::mozilla::SERVO_PREF_ENABLED_${property.gecko_pref_ident};
+        % else:
+            false;
+        % endif
+
+    let passes_pref_check =
+        % if property.experimental and product == "servo":
+            PREFS.get("${property.experimental}").as_boolean().unwrap_or(false);
+        % elif product == "gecko":
+            {
+                let id = structs::${helpers.to_nscsspropertyid(property.ident)};
+                unsafe { bindings::Gecko_PropertyId_IsPrefEnabled(id) }
+            };
+        % else:
+            true;
+        % endif
+
+    % if property.internal:
+        if context.stylesheet_origin != Origin::UserAgent {
+            if is_experimental {
+                if !passes_pref_check {
+                    return Err(PropertyDeclarationParseError::ExperimentalProperty);
+                }
+            } else {
+                return Err(PropertyDeclarationParseError::UnknownProperty);
             }
         }
+    % else:
+        if is_experimental && !passes_pref_check {
+            return Err(PropertyDeclarationParseError::ExperimentalProperty);
+        }
     % endif
 </%def>
 
@@ -1418,18 +1434,13 @@ impl PropertyDeclaration {
                                 return Err(PropertyDeclarationParseError::AnimationPropertyInKeyframeBlock)
                             }
                         % endif
-                        % if property.internal:
-                            if context.stylesheet_origin != Origin::UserAgent {
-                                return Err(PropertyDeclarationParseError::UnknownProperty)
-                            }
-                        % endif
                         % if not property.allowed_in_page_rule:
                             if rule_type == CssRuleType::Page {
                                 return Err(PropertyDeclarationParseError::NotAllowedInPageRule)
                             }
                         % endif
 
-                        ${property_pref_check(property)}
+                        ${property_exposure_check(property)}
 
                         match longhands::${property.ident}::parse_declared(context, input) {
                             Ok(value) => {
@@ -1452,18 +1463,13 @@ impl PropertyDeclaration {
                             return Err(PropertyDeclarationParseError::AnimationPropertyInKeyframeBlock)
                         }
                     % endif
-                    % if shorthand.internal:
-                        if context.stylesheet_origin != Origin::UserAgent {
-                            return Err(PropertyDeclarationParseError::UnknownProperty)
-                        }
-                    % endif
                     % if not shorthand.allowed_in_page_rule:
                         if rule_type == CssRuleType::Page {
                             return Err(PropertyDeclarationParseError::NotAllowedInPageRule)
                         }
                     % endif
 
-                    ${property_pref_check(shorthand)}
+                    ${property_exposure_check(shorthand)}
 
                     match input.try(|i| CSSWideKeyword::parse(context, i)) {
                         Ok(keyword) => {