diff --git a/servo/components/style/properties/longhand/list.mako.rs b/servo/components/style/properties/longhand/list.mako.rs
index 12a73d0466eb..c478dc6f98c7 100644
--- a/servo/components/style/properties/longhand/list.mako.rs
+++ b/servo/components/style/properties/longhand/list.mako.rs
@@ -15,31 +15,29 @@ ${helpers.single_keyword("list-style-position", "outside inside", animation_valu
 //     decimal-leading-zero, armenian, upper-armenian, lower-armenian, georgian, lower-roman,
 //     upper-roman
 //
-// TODO(bholley): Missing quite a few gecko properties here as well.
-//
-// In gecko, {upper,lower}-{roman,alpha} are implemented as @counter-styles in the
-// UA, however they can also be set from pres attrs. When @counter-style is supported
-// we may need to look into this and handle these differently.
-//
 // [1]: http://dev.w3.org/csswg/css-counter-styles/
 % if product == "servo":
-    ${helpers.single_keyword("list-style-type", """
-        disc none circle square decimal disclosure-open disclosure-closed lower-alpha upper-alpha
+    ${helpers.single_keyword(
+        "list-style-type",
+        """disc none circle square decimal disclosure-open disclosure-closed lower-alpha upper-alpha
         arabic-indic bengali cambodian cjk-decimal devanagari gujarati gurmukhi kannada khmer lao
         malayalam mongolian myanmar oriya persian telugu thai tibetan cjk-earthly-branch
         cjk-heavenly-stem lower-greek hiragana hiragana-iroha katakana katakana-iroha""",
         animation_value_type="discrete",
         spec="https://drafts.csswg.org/css-lists/#propdef-list-style-type",
-        servo_restyle_damage="rebuild_and_reflow")}
+        servo_restyle_damage="rebuild_and_reflow",
+    )}
 % else:
-    ${helpers.predefined_type("list-style-type",
-                              "ListStyleType",
-                              "computed::ListStyleType::disc()",
-                              initial_specified_value="specified::ListStyleType::disc()",
-                              animation_value_type="discrete",
-                              boxed=True,
-                              spec="https://drafts.csswg.org/css-lists/#propdef-list-style-type",
-                              servo_restyle_damage="rebuild_and_reflow")}
+    ${helpers.predefined_type(
+        "list-style-type",
+        "ListStyleType",
+        "computed::ListStyleType::disc()",
+        initial_specified_value="specified::ListStyleType::disc()",
+        animation_value_type="discrete",
+        boxed=True,
+        spec="https://drafts.csswg.org/css-lists/#propdef-list-style-type",
+        servo_restyle_damage="rebuild_and_reflow",
+    )}
 % endif
 
 ${helpers.predefined_type("list-style-image",
diff --git a/servo/components/style/values/generics/transform.rs b/servo/components/style/values/generics/transform.rs
index 504198ab89c6..7584e5cdf489 100644
--- a/servo/components/style/values/generics/transform.rs
+++ b/servo/components/style/values/generics/transform.rs
@@ -80,12 +80,13 @@ pub struct TransformOrigin<H, V, Depth> {
 /// A generic timing function.
 ///
 /// <https://drafts.csswg.org/css-timing-1/#single-timing-function-production>
-#[derive(Clone, Copy, Debug, MallocSizeOf, PartialEq)]
+#[derive(Clone, Copy, Debug, MallocSizeOf, PartialEq, ToCss)]
 pub enum TimingFunction<Integer, Number> {
     /// `linear | ease | ease-in | ease-out | ease-in-out`
     Keyword(TimingKeyword),
     /// `cubic-bezier(<number>, <number>, <number>, <number>)`
     #[allow(missing_docs)]
+    #[css(comma, function)]
     CubicBezier {
         x1: Number,
         y1: Number,
@@ -93,8 +94,10 @@ pub enum TimingFunction<Integer, Number> {
         y2: Number,
     },
     /// `step-start | step-end | steps(<integer>, [ start | end ]?)`
-    Steps(Integer, StepPosition),
+    #[css(comma, function)]
+    Steps(Integer, #[css(skip_if = "is_end")] StepPosition),
     /// `frames(<integer>)`
+    #[css(comma, function)]
     Frames(Integer),
 }
 
@@ -119,6 +122,11 @@ pub enum StepPosition {
     End,
 }
 
+#[inline]
+fn is_end(position: &StepPosition) -> bool {
+    *position == StepPosition::End
+}
+
 impl<H, V, D> TransformOrigin<H, V, D> {
     /// Returns a new transform origin.
     pub fn new(horizontal: H, vertical: V, depth: D) -> Self {
@@ -138,51 +146,6 @@ impl<Integer, Number> TimingFunction<Integer, Number> {
     }
 }
 
-impl<Integer, Number> ToCss for TimingFunction<Integer, Number>
-where
-    Integer: ToCss,
-    Number: ToCss,
-{
-    fn to_css<W>(&self, dest: &mut CssWriter<W>) -> fmt::Result
-    where
-        W: Write,
-    {
-        match *self {
-            TimingFunction::Keyword(keyword) => keyword.to_css(dest),
-            TimingFunction::CubicBezier {
-                ref x1,
-                ref y1,
-                ref x2,
-                ref y2,
-            } => {
-                dest.write_str("cubic-bezier(")?;
-                x1.to_css(dest)?;
-                dest.write_str(", ")?;
-                y1.to_css(dest)?;
-                dest.write_str(", ")?;
-                x2.to_css(dest)?;
-                dest.write_str(", ")?;
-                y2.to_css(dest)?;
-                dest.write_str(")")
-            },
-            TimingFunction::Steps(ref intervals, position) => {
-                dest.write_str("steps(")?;
-                intervals.to_css(dest)?;
-                if position != StepPosition::End {
-                    dest.write_str(", ")?;
-                    position.to_css(dest)?;
-                }
-                dest.write_str(")")
-            },
-            TimingFunction::Frames(ref frames) => {
-                dest.write_str("frames(")?;
-                frames.to_css(dest)?;
-                dest.write_str(")")
-            },
-        }
-    }
-}
-
 impl TimingKeyword {
     /// Returns the keyword as a quadruplet of Bezier point coordinates
     /// `(x1, y1, x2, y2)`.
diff --git a/servo/components/style_derive/to_css.rs b/servo/components/style_derive/to_css.rs
index a0197dedc86c..f94da49cfaa6 100644
--- a/servo/components/style_derive/to_css.rs
+++ b/servo/components/style_derive/to_css.rs
@@ -5,7 +5,7 @@
 use cg::{self, WhereClause};
 use darling::util::Override;
 use quote::{ToTokens, Tokens};
-use syn::{self, Data};
+use syn::{self, Data, Path};
 use synstructure::{BindingInfo, Structure, VariantInfo};
 
 pub fn derive(input: syn::DeriveInput) -> Tokens {
@@ -79,7 +79,6 @@ fn derive_variant_arm(
 
     let mut expr = if let Some(keyword) = variant_attrs.keyword {
         assert!(bindings.is_empty());
-        let keyword = keyword.to_string();
         quote! {
             ::std::fmt::Write::write_str(dest, #keyword)
         }
@@ -129,7 +128,15 @@ fn derive_variant_fields_expr(
         if !attrs.ignore_bound {
             where_clause.add_trait_bound(&first.ast().ty);
         }
-        return quote! { ::style_traits::ToCss::to_css(#first, dest) };
+        let mut expr = quote! { ::style_traits::ToCss::to_css(#first, dest) };
+        if let Some(condition) = attrs.skip_if {
+            expr = quote! {
+                if !#condition(#first) {
+                    #expr
+                }
+            }
+        }
+        return expr;
     }
 
     let mut expr = derive_single_field_expr(first, attrs, where_clause);
@@ -149,7 +156,7 @@ fn derive_single_field_expr(
     attrs: CssFieldAttrs,
     where_clause: &mut WhereClause,
 ) -> Tokens {
-    if attrs.iterable {
+    let mut expr = if attrs.iterable {
         if let Some(if_empty) = attrs.if_empty {
             return quote! {
                 {
@@ -174,7 +181,17 @@ fn derive_single_field_expr(
             where_clause.add_trait_bound(&field.ast().ty);
         }
         quote! { writer.item(#field)?; }
+    };
+
+    if let Some(condition) = attrs.skip_if {
+        expr = quote! {
+            if !#condition(#field) {
+                #expr
+            }
+        }
     }
+
+    expr
 }
 
 #[darling(attributes(css), default)]
@@ -204,4 +221,5 @@ struct CssFieldAttrs {
     ignore_bound: bool,
     iterable: bool,
     skip: bool,
+    skip_if: Option<Path>,
 }
diff --git a/servo/components/style_traits/values.rs b/servo/components/style_traits/values.rs
index aad75d51b8a7..d6bd05b7b40e 100644
--- a/servo/components/style_traits/values.rs
+++ b/servo/components/style_traits/values.rs
@@ -34,6 +34,9 @@ use std::fmt::{self, Write};
 ///   dimension token, like: <member><identifier>;
 /// * if `#[css(skip)]` is found on a field, the `ToCss` call for that field
 ///   is skipped;
+/// * if `#[css(skip_if = "function")]` is found on a field, the `ToCss` call
+///   for that field is skipped if `function` returns true. This function is
+///   provided the field as an argument;
 /// * finally, one can put `#[css(derive_debug)]` on the whole type, to
 ///   implement `Debug` by a single call to `ToCss::to_css`.
 pub trait ToCss {