Allow empty media query list

And make empty list the default value of MediaList.

This commit also removes Option wrapper of Stylesheet.media because
a stylesheet should always have an associated media query list.

components/style/media_queries.rs

@@ -31,6 +31,12 @@ impl ToCss for MediaList {
     }
 }
 
+impl Default for MediaList {
+    fn default() -> MediaList {
+        MediaList { media_queries: vec![] }
+    }
+}
+
 #[derive(PartialEq, Eq, Copy, Clone, Debug)]
 #[cfg_attr(feature = "servo", derive(HeapSizeOf))]
 pub enum Range<T> {
@@ -253,8 +259,8 @@ impl MediaQuery {
 }
 
 pub fn parse_media_query_list(input: &mut Parser) -> MediaList {
-    let queries = if input.is_exhausted() {
-        vec![MediaQuery::new(None, MediaQueryType::All, vec!())]
+    if input.is_exhausted() {
+        Default::default()
     } else {
         let mut media_queries = vec![];
         loop {
@@ -269,17 +275,17 @@ pub fn parse_media_query_list(input: &mut Parser) -> MediaList {
                 Err(()) => break,
             }
         }
-        media_queries
-    };
-    MediaList { media_queries: queries }
+        MediaList { media_queries: media_queries }
+    }
 }
 
 impl MediaList {
     pub fn evaluate(&self, device: &Device) -> bool {
         let viewport_size = device.au_viewport_size();
 
-        // Check if any queries match (OR condition)
-        self.media_queries.iter().any(|mq| {
+        // Check if it is an empty media query list or any queries match (OR condition)
+        // https://drafts.csswg.org/mediaqueries-4/#mq-list
+        self.media_queries.is_empty() || self.media_queries.iter().any(|mq| {
             // Check if media matches. Unknown media never matches.
             let media_match = match mq.media_type {
                 MediaQueryType::MediaType(MediaType::Unknown(_)) => false,

components/style/stylesheets.rs

@@ -45,8 +45,8 @@ pub struct Stylesheet {
     /// List of rules in the order they were found (important for
     /// cascading order)
     pub rules: Vec<CSSRule>,
-    /// List of media associated with the Stylesheet, if any.
-    pub media: Option<MediaList>,
+    /// List of media associated with the Stylesheet.
+    pub media: MediaList,
     pub origin: Origin,
     pub dirty_on_viewport_size_change: bool,
 }
@@ -181,14 +181,14 @@ impl Stylesheet {
         Stylesheet {
             origin: origin,
             rules: rules,
-            media: None,
+            media: Default::default(),
             dirty_on_viewport_size_change:
                 input.seen_viewport_percentages(),
         }
     }
 
     /// Set the MediaList associated with the style-sheet.
-    pub fn set_media(&mut self, media: Option<MediaList>) {
+    pub fn set_media(&mut self, media: MediaList) {
         self.media = media;
     }
 
@@ -197,7 +197,7 @@ impl Stylesheet {
     ///
     /// Always true if no associated MediaList exists.
     pub fn is_effective_for_device(&self, device: &Device) -> bool {
-        self.media.as_ref().map_or(true, |ref media| media.evaluate(device))
+        self.media.evaluate(device)
     }
 
     /// Return an iterator over the effective rules within the style-sheet, as