diff --git a/components/style/properties/data.py b/components/style/properties/data.py
index c935e00a79f..8db09e6fe81 100644
--- a/components/style/properties/data.py
+++ b/components/style/properties/data.py
@@ -140,7 +140,7 @@ class Longhand(object):
                  need_clone=False, need_index=False, gecko_ffi_name=None, depend_on_viewport_size=False,
                  allowed_in_keyframe_block=True, complex_color=False, cast_type='u8',
                  has_uncacheable_values=False, logical=False, alias=None, extra_prefixes=None, boxed=False,
-                 flags=None, allowed_in_page_rule=False):
+                 flags=None, allowed_in_page_rule=False, allow_quirks=False):
         self.name = name
         if not spec:
             raise TypeError("Spec should be specified for %s" % name)
@@ -166,6 +166,7 @@ class Longhand(object):
         self.boxed = arg_to_bool(boxed)
         self.flags = flags.split() if flags else []
         self.allowed_in_page_rule = arg_to_bool(allowed_in_page_rule)
+        self.allow_quirks = allow_quirks
 
         # https://drafts.csswg.org/css-animations/#keyframes
         # > The <declaration-list> inside of <keyframe-block> accepts any CSS property
diff --git a/components/style/properties/helpers.mako.rs b/components/style/properties/helpers.mako.rs
index 5cfe95eec4d..6563b97df15 100644
--- a/components/style/properties/helpers.mako.rs
+++ b/components/style/properties/helpers.mako.rs
@@ -375,7 +375,11 @@
                     parse(context, input).map(|result| Box::new(result))
                 % else:
                                    -> Result<SpecifiedValue, ()> {
-                    parse(context, input)
+                    % if property.allow_quirks:
+                        parse_quirky(context, input, specified::AllowQuirks::Yes)
+                    % else:
+                        parse(context, input)
+                    % endif
                 % endif
             }
             pub fn parse_declared(context: &ParserContext, input: &mut Parser)
diff --git a/components/style/properties/longhand/font.mako.rs b/components/style/properties/longhand/font.mako.rs
index 6bdc67037d3..02df590ee9d 100644
--- a/components/style/properties/longhand/font.mako.rs
+++ b/components/style/properties/longhand/font.mako.rs
@@ -550,14 +550,14 @@ ${helpers.single_keyword_system("font-variant-caps",
 </%helpers:longhand>
 
 <%helpers:longhand name="font-size" need_clone="True" animation_value_type="ComputedValue"
-                   spec="https://drafts.csswg.org/css-fonts/#propdef-font-size">
+                   allow_quirks="True" spec="https://drafts.csswg.org/css-fonts/#propdef-font-size">
     use app_units::Au;
     use properties::longhands::system_font::SystemFont;
     use properties::style_structs::Font;
     use std::fmt;
     use style_traits::ToCss;
     use values::{FONT_MEDIUM_PX, HasViewportPercentage};
-    use values::specified::{FontRelativeLength, LengthOrPercentage, Length};
+    use values::specified::{AllowQuirks, FontRelativeLength, LengthOrPercentage, Length};
     use values::specified::{NoCalcLength, Percentage};
     use values::specified::length::FontBaseSize;
 
@@ -843,9 +843,19 @@ ${helpers.single_keyword_system("font-variant-caps",
                 ))
         }
     }
+
     /// <length> | <percentage> | <absolute-size> | <relative-size>
     pub fn parse(context: &ParserContext, input: &mut Parser) -> Result<SpecifiedValue, ()> {
-        if let Ok(lop) = input.try(|i| specified::LengthOrPercentage::parse_non_negative(context, i)) {
+        parse_quirky(context, input, AllowQuirks::No)
+    }
+
+    /// Parses a font-size, with quirks.
+    pub fn parse_quirky(context: &ParserContext,
+                        input: &mut Parser,
+                        allow_quirks: AllowQuirks)
+                        -> Result<SpecifiedValue, ()> {
+        use self::specified::LengthOrPercentage;
+        if let Ok(lop) = input.try(|i| LengthOrPercentage::parse_non_negative_quirky(context, i, allow_quirks)) {
             return Ok(SpecifiedValue::Length(lop))
         }
 
diff --git a/components/style/values/specified/length.rs b/components/style/values/specified/length.rs
index 142b44e1861..6a66e658aef 100644
--- a/components/style/values/specified/length.rs
+++ b/components/style/values/specified/length.rs
@@ -1204,7 +1204,16 @@ impl LengthOrPercentage {
     /// Parse a non-negative length.
     #[inline]
     pub fn parse_non_negative(context: &ParserContext, input: &mut Parser) -> Result<LengthOrPercentage, ()> {
-        Self::parse_internal(context, input, AllowedLengthType::NonNegative, AllowQuirks::No)
+        Self::parse_non_negative_quirky(context, input, AllowQuirks::No)
+    }
+
+    /// Parse a non-negative length, with quirks.
+    #[inline]
+    pub fn parse_non_negative_quirky(context: &ParserContext,
+                                     input: &mut Parser,
+                                     allow_quirks: AllowQuirks)
+                                     -> Result<LengthOrPercentage, ()> {
+        Self::parse_internal(context, input, AllowedLengthType::NonNegative, allow_quirks)
     }
 
     /// Parse a length, treating dimensionless numbers as pixels
diff --git a/tests/wpt/metadata/quirks-mode/unitless-length.html.ini b/tests/wpt/metadata/quirks-mode/unitless-length.html.ini
index 6b916d49a89..95010cb9d61 100644
--- a/tests/wpt/metadata/quirks-mode/unitless-length.html.ini
+++ b/tests/wpt/metadata/quirks-mode/unitless-length.html.ini
@@ -12,9 +12,6 @@
     expected:
       if os == "mac": FAIL
 
-  [font-size: 1 (quirks)]
-    expected: FAIL
-
   [height: 1 (quirks)]
     expected: FAIL
 
@@ -96,9 +93,6 @@
     expected:
       if os == "mac": FAIL
 
-  [font-size: +1 (quirks)]
-    expected: FAIL
-
   [height: +1 (quirks)]
     expected: FAIL
 
@@ -231,9 +225,6 @@
     expected:
       if os == "mac": FAIL
 
-  [font-size: 1.5 (quirks)]
-    expected: FAIL
-
   [height: 1.5 (quirks)]
     expected: FAIL
 
@@ -315,9 +306,6 @@
     expected:
       if os == "mac": FAIL
 
-  [font-size: +1.5 (quirks)]
-    expected: FAIL
-
   [height: +1.5 (quirks)]
     expected: FAIL