diff --git a/components/style/matching.rs b/components/style/matching.rs
index c26ed839986..25349b5457f 100644
--- a/components/style/matching.rs
+++ b/components/style/matching.rs
@@ -13,8 +13,9 @@ use data::{ComputedStyle, ElementData, RestyleData};
 use dom::{TElement, TNode};
 use font_metrics::FontMetricsProvider;
 use log::LogLevel::Trace;
+use properties::{ALLOW_SET_ROOT_FONT_SIZE, SKIP_ROOT_AND_ITEM_BASED_DISPLAY_FIXUP};
 use properties::{AnimationRules, CascadeFlags, ComputedValues};
-use properties::{SKIP_ROOT_AND_ITEM_BASED_DISPLAY_FIXUP, VISITED_DEPENDENT_ONLY, cascade};
+use properties::{VISITED_DEPENDENT_ONLY, cascade};
 use properties::longhands::display::computed_value as display;
 use restyle_hints::{RESTYLE_CSS_ANIMATIONS, RESTYLE_CSS_TRANSITIONS, RestyleReplacements};
 use restyle_hints::{RESTYLE_STYLE_ATTRIBUTE, RESTYLE_SMIL};
@@ -256,6 +257,9 @@ trait PrivateMatchMethods: TElement {
         if cascade_visited.visited_dependent_only() {
             cascade_flags.insert(VISITED_DEPENDENT_ONLY);
         }
+        if !self.is_native_anonymous() {
+            cascade_flags.insert(ALLOW_SET_ROOT_FONT_SIZE);
+        }
 
         // Grab the inherited values.
         let parent_el;
diff --git a/components/style/properties/properties.mako.rs b/components/style/properties/properties.mako.rs
index 5036055d379..d2eab593f19 100644
--- a/components/style/properties/properties.mako.rs
+++ b/components/style/properties/properties.mako.rs
@@ -2483,6 +2483,15 @@ bitflags! {
         const SKIP_ROOT_AND_ITEM_BASED_DISPLAY_FIXUP = 0x02,
         /// Whether to only cascade properties that are visited dependent.
         const VISITED_DEPENDENT_ONLY = 0x04,
+        /// Should we modify the device's root font size
+        /// when computing the root?
+        ///
+        /// Not set for native anonymous content since some NAC
+        /// form their own root, but share the device.
+        ///
+        /// ::backdrop and all NAC will resolve rem units against
+        /// the toplevel root element now.
+        const ALLOW_SET_ROOT_FONT_SIZE = 0x08,
     }
 }
 
@@ -2773,7 +2782,7 @@ pub fn apply_declarations<'a, F, I>(device: &Device,
             % endif
             }
 
-            if is_root_element {
+            if is_root_element && flags.contains(ALLOW_SET_ROOT_FONT_SIZE) {
                 let s = context.style.get_font().clone_font_size();
                 context.device.set_root_font_size(s);
             }