diff --git a/components/style/supports.rs b/components/style/supports.rs
index bbbfec9f4fc5..455772a9eedc 100644
--- a/components/style/supports.rs
+++ b/components/style/supports.rs
@@ -44,7 +44,7 @@ impl SupportsCondition {
         let (keyword, wrapper) = match input.next() {
             Err(()) => {
                 // End of input
-                return Ok(SupportsCondition::Parenthesized(Box::new(in_parens)))
+                return Ok(in_parens)
             }
             Ok(Token::Ident(ident)) => {
                 match_ignore_ascii_case! { ident,
@@ -71,6 +71,9 @@ impl SupportsCondition {
 
     /// https://drafts.csswg.org/css-conditional-3/#supports_condition_in_parens
     fn parse_in_parens(input: &mut Parser) -> Result<SupportsCondition, ()> {
+        // Whitespace is normally taken care of in `Parser::next`,
+        // but we want to not include it in `pos` for the SupportsCondition::FutureSyntax cases.
+        while input.try(Parser::expect_whitespace).is_ok() {}
         let pos = input.position();
         match input.next()? {
             Token::ParenthesisBlock => {
@@ -106,9 +109,11 @@ impl SupportsCondition {
 /// supports_condition | declaration
 /// https://drafts.csswg.org/css-conditional/#dom-css-supports-conditiontext-conditiontext
 pub fn parse_condition_or_declaration(input: &mut Parser) -> Result<SupportsCondition, ()> {
-    input.try(SupportsCondition::parse).or_else(|()| {
+    if let Ok(condition) = input.try(SupportsCondition::parse) {
+        Ok(SupportsCondition::Parenthesized(Box::new(condition)))
+    } else {
         Declaration::parse(input).map(SupportsCondition::Declaration)
-    })
+    }
 }
 
 impl ToCss for SupportsCondition {
@@ -116,9 +121,8 @@ impl ToCss for SupportsCondition {
         where W: fmt::Write {
         match *self {
             SupportsCondition::Not(ref cond) => {
-                dest.write_str("not (")?;
-                cond.to_css(dest)?;
-                dest.write_str(")")
+                dest.write_str("not ")?;
+                cond.to_css(dest)
             }
             SupportsCondition::Parenthesized(ref cond) => {
                 dest.write_str("(")?;
@@ -132,9 +136,7 @@ impl ToCss for SupportsCondition {
                         dest.write_str(" and ")?;
                     }
                     first = false;
-                    dest.write_str("(")?;
                     cond.to_css(dest)?;
-                    dest.write_str(")")?;
                 }
                 Ok(())
             }
@@ -145,13 +147,15 @@ impl ToCss for SupportsCondition {
                         dest.write_str(" or ")?;
                     }
                     first = false;
-                    dest.write_str("(")?;
                     cond.to_css(dest)?;
-                    dest.write_str(")")?;
                 }
                 Ok(())
             }
-            SupportsCondition::Declaration(ref decl) => decl.to_css(dest),
+            SupportsCondition::Declaration(ref decl) => {
+                dest.write_str("(")?;
+                decl.to_css(dest)?;
+                dest.write_str(")")
+            }
             SupportsCondition::FutureSyntax(ref s) => dest.write_str(&s),
         }
     }
diff --git a/tests/unit/style/parsing/mod.rs b/tests/unit/style/parsing/mod.rs
index 1fede915771e..2521f5a9822d 100644
--- a/tests/unit/style/parsing/mod.rs
+++ b/tests/unit/style/parsing/mod.rs
@@ -39,6 +39,25 @@ macro_rules! assert_roundtrip_with_context {
     }
 }
 
+macro_rules! assert_roundtrip {
+    ($fun:expr, $string:expr) => {
+        assert_roundtrip!($fun, $string, $string);
+    };
+    ($fun:expr,$input:expr, $output:expr) => {
+        let mut parser = Parser::new($input);
+        let parsed = $fun(&mut parser)
+                     .expect(&format!("Failed to parse {}", $input));
+        let serialized = ToCss::to_css_string(&parsed);
+        assert_eq!(serialized, $output);
+
+        let mut parser = Parser::new(&serialized);
+        let re_parsed = $fun(&mut parser)
+                        .expect(&format!("Failed to parse serialization {}", $input));
+        let re_serialized = ToCss::to_css_string(&re_parsed);
+        assert_eq!(serialized, re_serialized);
+    }
+}
+
 macro_rules! parse_longhand {
     ($name:ident, $s:expr) => {{
         let url = ::servo_url::ServoUrl::parse("http://localhost").unwrap();
@@ -58,3 +77,4 @@ mod inherited_text;
 mod mask;
 mod position;
 mod selectors;
+mod supports;
diff --git a/tests/unit/style/parsing/supports.rs b/tests/unit/style/parsing/supports.rs
new file mode 100644
index 000000000000..b38f1b09bc0d
--- /dev/null
+++ b/tests/unit/style/parsing/supports.rs
@@ -0,0 +1,14 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+use cssparser::Parser;
+use style::supports::SupportsCondition;
+use style_traits::ToCss;
+
+#[test]
+fn test_supports_condition() {
+    assert_roundtrip!(SupportsCondition::parse, "(margin: 1px)");
+    assert_roundtrip!(SupportsCondition::parse, "not (--be: to be)");
+    assert_roundtrip!(SupportsCondition::parse, "(color: blue) and future-extension(4)");
+}