--- a/servo/components/style/properties/longhand/box.mako.rs
+++ b/servo/components/style/properties/longhand/box.mako.rs
@@ -98,58 +98,23 @@
 </%helpers:longhand>
 
 ${helpers.single_keyword("-moz-top-layer", "none top",
                          gecko_constant_prefix="NS_STYLE_TOP_LAYER",
                          gecko_ffi_name="mTopLayer", need_clone=True,
                          products="gecko", animatable=False, internal=True,
                          spec="Internal (not web-exposed)")}
 
-<%helpers:single_keyword_computed name="position"
-                                  values="static absolute relative fixed"
-                                  need_clone="True"
-                                  extra_gecko_values="sticky"
-                                  animatable="False"
-                                  creates_stacking_context="True"
-                                  abspos_cb="True"
-                                  spec="https://drafts.csswg.org/css-position/#position-property">
-    impl SpecifiedValue {
-        pub fn is_absolutely_positioned_style(&self) -> bool {
-            matches!(*self, SpecifiedValue::absolute | SpecifiedValue::fixed)
-        }
-    }
-
-    use values::HasViewportPercentage;
-    no_viewport_percentage!(SpecifiedValue);
-    impl ToComputedValue for SpecifiedValue {
-        type ComputedValue = computed_value::T;
-
-        #[inline]
-        fn to_computed_value(&self, _context: &Context) -> computed_value::T {
-            % if product == "gecko":
-                // https://fullscreen.spec.whatwg.org/#new-stacking-layer
-                // Any position value other than 'absolute' and 'fixed' are
-                // computed to 'absolute' if the element is in a top layer.
-                if !self.is_absolutely_positioned_style() &&
-                   matches!(_context.style().get_box().clone__moz_top_layer(),
-                            longhands::_moz_top_layer::SpecifiedValue::top) {
-                    SpecifiedValue::absolute
-                } else {
-                    *self
-                }
-            % else:
-                *self
-            % endif
-        }
-        #[inline]
-        fn from_computed_value(computed: &computed_value::T) -> SpecifiedValue {
-            *computed
-        }
-    }
-</%helpers:single_keyword_computed>
+${helpers.single_keyword("position", "static absolute relative fixed",
+                         need_clone="True",
+                         extra_gecko_values="sticky",
+                         animatable="False",
+                         creates_stacking_context="True",
+                         abspos_cb="True",
+                         spec="https://drafts.csswg.org/css-position/#position-property")}
 
 <%helpers:single_keyword_computed name="float"
                                   values="none left right"
                                   // https://drafts.csswg.org/css-logical-props/#float-clear
                                   extra_specified="inline-start inline-end"
                                   needs_conversion="True"
                                   animatable="False"
                                   need_clone="True"
@@ -159,30 +124,26 @@
                                   spec="https://drafts.csswg.org/css-box/#propdef-float">
     use values::HasViewportPercentage;
     no_viewport_percentage!(SpecifiedValue);
     impl ToComputedValue for SpecifiedValue {
         type ComputedValue = computed_value::T;
 
         #[inline]
         fn to_computed_value(&self, context: &Context) -> computed_value::T {
-            if context.style().get_box().clone_position().is_absolutely_positioned_style() {
-                computed_value::T::none
-            } else {
-                let ltr = context.style().writing_mode.is_bidi_ltr();
-                // https://drafts.csswg.org/css-logical-props/#float-clear
-                match *self {
-                    SpecifiedValue::inline_start if ltr => computed_value::T::left,
-                    SpecifiedValue::inline_start => computed_value::T::right,
-                    SpecifiedValue::inline_end if ltr => computed_value::T::right,
-                    SpecifiedValue::inline_end => computed_value::T::left,
-                    % for value in "none left right".split():
-                        SpecifiedValue::${value} => computed_value::T::${value},
-                    % endfor
-                }
+            let ltr = context.style().writing_mode.is_bidi_ltr();
+            // https://drafts.csswg.org/css-logical-props/#float-clear
+            match *self {
+                SpecifiedValue::inline_start if ltr => computed_value::T::left,
+                SpecifiedValue::inline_start => computed_value::T::right,
+                SpecifiedValue::inline_end if ltr => computed_value::T::right,
+                SpecifiedValue::inline_end => computed_value::T::left,
+                % for value in "none left right".split():
+                    SpecifiedValue::${value} => computed_value::T::${value},
+                % endfor
             }
         }
         #[inline]
         fn from_computed_value(computed: &computed_value::T) -> SpecifiedValue {
             match *computed {
                 % for value in "none left right".split():
                     computed_value::T::${value} => SpecifiedValue::${value},
                 % endfor

--- a/servo/components/style/properties/properties.mako.rs
+++ b/servo/components/style/properties/properties.mako.rs
@@ -2045,18 +2045,16 @@ pub fn apply_declarations<'a, F, I>(devi
             // "early" properties then "other".
             //
             // Unfortunately, it’s not easy to check that this
             // classification is correct.
             let is_early_property = matches!(longhand_id,
                 LonghandId::FontSize |
                 LonghandId::FontFamily |
                 LonghandId::Color |
-                LonghandId::Position |
-                LonghandId::Float |
                 LonghandId::TextDecorationLine |
                 LonghandId::WritingMode |
                 LonghandId::Direction
                 % if product == 'gecko':
                     | LonghandId::TextOrientation
                     | LonghandId::AnimationName
                     | LonghandId::TransitionProperty
                 % endif
@@ -2089,19 +2087,34 @@ pub fn apply_declarations<'a, F, I>(devi
         % if category_to_cascade_now == "early":
             let writing_mode = get_writing_mode(context.style.get_inheritedbox());
             context.style.writing_mode = writing_mode;
         % endif
     % endfor
 
     let mut style = context.style;
 
-    let positioned = matches!(style.get_box().clone_position(),
+    let mut positioned = matches!(style.get_box().clone_position(),
         longhands::position::SpecifiedValue::absolute |
         longhands::position::SpecifiedValue::fixed);
+
+    // https://fullscreen.spec.whatwg.org/#new-stacking-layer
+    // Any position value other than 'absolute' and 'fixed' are
+    // computed to 'absolute' if the element is in a top layer.
+    % if product == "gecko":
+        if !positioned &&
+            matches!(style.get_box().clone__moz_top_layer(),
+                     longhands::_moz_top_layer::SpecifiedValue::top) {
+            positioned = true;
+            style.mutate_box().set_position(longhands::position::computed_value::T::absolute);
+        }
+    % endif
+
+    let positioned = positioned; // To ensure it's not mutated further.
+
     let floated = style.get_box().clone_float() != longhands::float::computed_value::T::none;
     let is_item = matches!(context.layout_parent_style.get_box().clone_display(),
         % if product == "gecko":
         computed_values::display::T::grid |
         computed_values::display::T::inline_grid |
         % endif
         computed_values::display::T::flex |
         computed_values::display::T::inline_flex);