style: Fix rem computation on the root element.

Bug: 1375930
Reviewed-By: heycam
MozReview-Commit-ID: DK98SS1w5nO

components/style/values/specified/length.rs

@@ -112,7 +112,6 @@ impl FontRelativeLength {
 
         let reference_font_size = base_size.resolve(context);
 
-        let root_font_size = context.device.root_font_size();
         match *self {
             FontRelativeLength::Em(length) => reference_font_size.scale_by(length),
             FontRelativeLength::Ex(length) => {
@@ -149,7 +148,19 @@ impl FontRelativeLength {
                     }
                 }
             }
-            FontRelativeLength::Rem(length) => root_font_size.scale_by(length)
+            FontRelativeLength::Rem(length) => {
+                // https://drafts.csswg.org/css-values/#rem:
+                //
+                //     When specified on the font-size property of the root
+                //     element, the rem units refer to the property’s initial
+                //     value.
+                //
+                if context.is_root_element {
+                    reference_font_size.scale_by(length)
+                } else {
+                    context.device.root_font_size().scale_by(length)
+                }
+            }
         }
     }
 }