From 62bea2803101adb2f1fc08afc51455951a986c35 Mon Sep 17 00:00:00 2001
From: Manish Goregaokar <manishsmail@gmail.com>
Date: Mon, 2 Jan 2017 00:11:27 -0800
Subject: [PATCH] Add CSS.supports()

---
 components/script/dom/css.rs             | 28 ++++++++++++++++++++++--
 components/script/dom/webidls/CSS.webidl |  6 +++++
 components/style/supports.rs             |  4 ++--
 3 files changed, 34 insertions(+), 4 deletions(-)

diff --git a/components/script/dom/css.rs b/components/script/dom/css.rs
index 2e04aed50bc3..bf264f143228 100644
--- a/components/script/dom/css.rs
+++ b/components/script/dom/css.rs
@@ -2,11 +2,14 @@
  * 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::serialize_identifier;
+use cssparser::{Parser, serialize_identifier};
+use dom::bindings::codegen::Bindings::WindowBinding::WindowBinding::WindowMethods;
 use dom::bindings::error::Fallible;
 use dom::bindings::reflector::Reflector;
 use dom::bindings::str::DOMString;
 use dom::window::Window;
+use style::parser::ParserContext;
+use style::supports::{Declaration, parse_condition_or_declaration};
 
 #[dom_struct]
 pub struct CSS {
@@ -14,10 +17,31 @@ pub struct CSS {
 }
 
 impl CSS {
-    // http://dev.w3.org/csswg/cssom/#serialize-an-identifier
+    /// http://dev.w3.org/csswg/cssom/#serialize-an-identifier
     pub fn Escape(_: &Window, ident: DOMString) -> Fallible<DOMString> {
         let mut escaped = String::new();
         serialize_identifier(&ident, &mut escaped).unwrap();
         Ok(DOMString::from(escaped))
     }
+
+    /// https://drafts.csswg.org/css-conditional/#dom-css-supports
+    pub fn Supports(win: &Window, property: DOMString, value: DOMString) -> bool {
+        let decl = Declaration { prop: property.into(), val: value.into() };
+        let url = win.Document().url();
+        let context = ParserContext::new_for_cssom(&url);
+        decl.eval(&context)
+    }
+
+    /// https://drafts.csswg.org/css-conditional/#dom-css-supports
+    pub fn Supports_(win: &Window, condition: DOMString) -> bool {
+        let mut input = Parser::new(&condition);
+        let cond = parse_condition_or_declaration(&mut input);
+        if let Ok(cond) = cond {
+            let url = win.Document().url();
+            let context = ParserContext::new_for_cssom(&url);
+            cond.eval(&context)
+        } else {
+            false
+        }
+    }
 }
diff --git a/components/script/dom/webidls/CSS.webidl b/components/script/dom/webidls/CSS.webidl
index 438c6814ccb2..5f4aa4f0befc 100644
--- a/components/script/dom/webidls/CSS.webidl
+++ b/components/script/dom/webidls/CSS.webidl
@@ -11,3 +11,9 @@ interface CSS {
   [Throws]
   static DOMString escape(DOMString ident);
 };
+
+// https://drafts.csswg.org/css-conditional-3/#the-css-interface
+partial interface CSS {
+  static boolean supports(DOMString property, DOMString value);
+  static boolean supports(DOMString conditionText);
+};
diff --git a/components/style/supports.rs b/components/style/supports.rs
index a373ad58b0be..bbbfec9f4fc5 100644
--- a/components/style/supports.rs
+++ b/components/style/supports.rs
@@ -161,9 +161,9 @@ impl ToCss for SupportsCondition {
 /// A possibly-invalid property declaration
 pub struct Declaration {
     /// The property name
-    prop: String,
+    pub prop: String,
     /// The property value
-    val: String,
+    pub val: String,
 }
 
 impl ToCss for Declaration {