3 files changed, 111 insertions, 1 deletions

diff --git a/components/style/values/computed/mod.rs b/components/style/values/computed/mod.rs
index e74386bfc42..c6b33d58748 100644
--- a/components/style/values/computed/mod.rs
+++ b/components/style/values/computed/mod.rs
@@ -7,7 +7,7 @@
 use {Atom, Namespace};
 use context::QuirksMode;
 use euclid::Size2D;
-use font_metrics::FontMetricsProvider;
+use font_metrics::{FontMetricsProvider, get_metrics_provider_for_product};
 use media_queries::Device;
 #[cfg(feature = "gecko")]
 use properties;
@@ -136,6 +136,36 @@ pub struct Context<'a> {
 }
 
 impl<'a> Context<'a> {
+    /// Creates a suitable context for media query evaluation, in which
+    /// font-relative units compute against the system_font, and executes `f`
+    /// with it.
+    pub fn for_media_query_evaluation<F, R>(
+        device: &Device,
+        quirks_mode: QuirksMode,
+        f: F,
+    ) -> R
+    where
+        F: FnOnce(&Context) -> R
+    {
+        let mut conditions = RuleCacheConditions::default();
+        let default_values = device.default_computed_values();
+        let provider = get_metrics_provider_for_product();
+
+        let context = Context {
+            is_root_element: false,
+            builder: StyleBuilder::for_derived_style(device, default_values, None, None),
+            font_metrics_provider: &provider,
+            cached_system_font: None,
+            in_media_query: true,
+            quirks_mode,
+            for_smil_animation: false,
+            for_non_inherited_property: None,
+            rule_cache_conditions: RefCell::new(&mut conditions),
+        };
+
+        f(&context)
+    }
+
     /// Whether the current element is the root element.
     pub fn is_root_element(&self) -> bool {
         self.is_root_element
diff --git a/components/style/values/specified/mod.rs b/components/style/values/specified/mod.rs
index 004910ae6b1..ffe4d7ba59b 100644
--- a/components/style/values/specified/mod.rs
+++ b/components/style/values/specified/mod.rs
@@ -76,6 +76,7 @@ pub mod length;
 pub mod percentage;
 pub mod position;
 pub mod rect;
+pub mod source_size_list;
 pub mod svg;
 pub mod table;
 pub mod text;
diff --git a/components/style/values/specified/source_size_list.rs b/components/style/values/specified/source_size_list.rs
new file mode 100644
index 00000000000..4a0c068ce78
--- /dev/null
+++ b/components/style/values/specified/source_size_list.rs
@@ -0,0 +1,79 @@
+/* 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/. */
+
+//! https://html.spec.whatwg.org/multipage/#source-size-list
+
+use app_units::Au;
+use cssparser::Parser;
+use media_queries::{Device, Expression as MediaExpression};
+use parser::{Parse, ParserContext};
+use selectors::context::QuirksMode;
+use style_traits::ParseError;
+use values::computed::{self, ToComputedValue};
+use values::specified::Length;
+
+/// A value for a `<source-size>`:
+///
+/// https://html.spec.whatwg.org/multipage/#source-size
+pub struct SourceSize {
+    condition: MediaExpression,
+    value: Length,
+}
+
+impl Parse for SourceSize {
+    fn parse<'i, 't>(
+        context: &ParserContext,
+        input: &mut Parser<'i, 't>,
+    ) -> Result<Self, ParseError<'i>> {
+        let condition = MediaExpression::parse(context, input)?;
+        let value = Length::parse_non_negative(context, input)?;
+
+        Ok(Self { condition, value })
+    }
+}
+
+/// A value for a `<source-size-list>`:
+///
+/// https://html.spec.whatwg.org/multipage/#source-size-list
+pub struct SourceSizeList {
+    source_sizes: Vec<SourceSize>,
+    value: Length,
+}
+
+impl SourceSizeList {
+    /// Evaluate this <source-size-list> to get the final viewport length.
+    pub fn evaluate(&self, device: &Device, quirks_mode: QuirksMode) -> Au {
+        let matching_source_size = self.source_sizes.iter().find(|source_size| {
+            source_size.condition.matches(device, quirks_mode)
+        });
+
+        computed::Context::for_media_query_evaluation(device, quirks_mode, |context| {
+            match matching_source_size {
+                Some(source_size) => {
+                    source_size.value.to_computed_value(context)
+                }
+                None => {
+                    self.value.to_computed_value(context)
+                }
+            }
+        }).into()
+    }
+}
+
+impl Parse for SourceSizeList {
+    fn parse<'i, 't>(
+        context: &ParserContext,
+        input: &mut Parser<'i, 't>,
+    ) -> Result<Self, ParseError<'i>> {
+        let source_sizes = input.try(|input| {
+            input.parse_comma_separated(|input| {
+                SourceSize::parse(context, input)
+            })
+        }).unwrap_or(vec![]);
+
+        let value = Length::parse_non_negative(context, input)?;
+
+        Ok(Self { source_sizes, value })
+    }
+}