diff --git a/components/style/font_metrics.rs b/components/style/font_metrics.rs
index 13a4e9e182b..259e5693942 100644
--- a/components/style/font_metrics.rs
+++ b/components/style/font_metrics.rs
@@ -18,6 +18,8 @@ pub struct FontMetrics {
     pub zero_advance_measure: Option<Length>,
     /// The cap-height of the font.
     pub cap_height: Option<Length>,
+    /// The ideographic-width of the font.
+    pub ic_width: Option<Length>,
     /// The ascent of the font (a value is always available for this).
     pub ascent: Length,
 }
@@ -28,6 +30,7 @@ impl Default for FontMetrics {
             x_height: None,
             zero_advance_measure: None,
             cap_height: None,
+            ic_width: None,
             ascent: Length::new(0.0),
         }
     }
diff --git a/components/style/values/generics/calc.rs b/components/style/values/generics/calc.rs
index d2bc2a85523..d9044bbb818 100644
--- a/components/style/values/generics/calc.rs
+++ b/components/style/values/generics/calc.rs
@@ -47,6 +47,7 @@ pub enum SortKey {
     Deg,
     Em,
     Ex,
+    Ic,
     Px,
     Rem,
     Sec,
diff --git a/components/style/values/specified/calc.rs b/components/style/values/specified/calc.rs
index d6997934554..f5448591f21 100644
--- a/components/style/values/specified/calc.rs
+++ b/components/style/values/specified/calc.rs
@@ -185,6 +185,7 @@ impl generic::CalcNodeLeaf for Leaf {
                     FontRelativeLength::Em(..) => SortKey::Em,
                     FontRelativeLength::Ex(..) => SortKey::Ex,
                     FontRelativeLength::Cap(..) => SortKey::Cap,
+                    FontRelativeLength::Ic(..) => SortKey::Ic,
                     FontRelativeLength::Rem(..) => SortKey::Rem,
                 },
                 NoCalcLength::ViewportPercentage(ref vp) => match *vp {
diff --git a/components/style/values/specified/length.rs b/components/style/values/specified/length.rs
index 6105155d81d..600d65a531c 100644
--- a/components/style/values/specified/length.rs
+++ b/components/style/values/specified/length.rs
@@ -59,6 +59,9 @@ pub enum FontRelativeLength {
     /// A "cap" value: https://drafts.csswg.org/css-values/#cap
     #[css(dimension)]
     Cap(CSSFloat),
+    /// An "ic" value: https://drafts.csswg.org/css-values/#ic
+    #[css(dimension)]
+    Ic(CSSFloat),
     /// A "rem" value: https://drafts.csswg.org/css-values/#rem
     #[css(dimension)]
     Rem(CSSFloat),
@@ -93,6 +96,7 @@ impl FontRelativeLength {
             FontRelativeLength::Ex(v) |
             FontRelativeLength::Ch(v) |
             FontRelativeLength::Cap(v) |
+            FontRelativeLength::Ic(v) |
             FontRelativeLength::Rem(v) => v == 0.,
         }
     }
@@ -103,6 +107,7 @@ impl FontRelativeLength {
             FontRelativeLength::Ex(v) |
             FontRelativeLength::Ch(v) |
             FontRelativeLength::Cap(v) |
+            FontRelativeLength::Ic(v) |
             FontRelativeLength::Rem(v) => v < 0.,
         }
     }
@@ -119,12 +124,13 @@ impl FontRelativeLength {
             (&Ex(one), &Ex(other)) => Ex(one + other),
             (&Ch(one), &Ch(other)) => Ch(one + other),
             (&Cap(one), &Cap(other)) => Cap(one + other),
+            (&Ic(one), &Ic(other)) => Ic(one + other),
             (&Rem(one), &Rem(other)) => Rem(one + other),
             // See https://github.com/rust-lang/rust/issues/68867. rustc isn't
             // able to figure it own on its own so we help.
             _ => unsafe {
                 match *self {
-                    Em(..) | Ex(..) | Ch(..) | Cap(..) | Rem(..) => {},
+                    Em(..) | Ex(..) | Ch(..) | Cap(..) | Ic(..) | Rem(..) => {},
                 }
                 debug_unreachable!("Forgot to handle unit in try_sum()")
             },
@@ -237,6 +243,20 @@ impl FontRelativeLength {
                 });
                 (reference_size, length)
             },
+            FontRelativeLength::Ic(length) => {
+                let metrics =
+                    query_font_metrics(context, base_size, FontMetricsOrientation::MatchContextPreferVertical);
+                let reference_size = metrics.ic_width.unwrap_or_else(|| {
+                    // https://drafts.csswg.org/css-values/#ic
+                    //
+                    //     In the cases where it is impossible or impractical to
+                    //     determine the ideographic advance measure, it must be
+                    //     assumed to be 1em.
+                    //
+                    reference_font_size
+                });
+                (reference_size, length)
+            },
             FontRelativeLength::Rem(length) => {
                 // https://drafts.csswg.org/css-values/#rem:
                 //
@@ -549,6 +569,7 @@ impl NoCalcLength {
             "ex" => NoCalcLength::FontRelative(FontRelativeLength::Ex(value)),
             "ch" => NoCalcLength::FontRelative(FontRelativeLength::Ch(value)),
             "cap" => NoCalcLength::FontRelative(FontRelativeLength::Cap(value)),
+            "ic" => NoCalcLength::FontRelative(FontRelativeLength::Ic(value)),
             "rem" => NoCalcLength::FontRelative(FontRelativeLength::Rem(value)),
             // viewport percentages
             "vw" if !context.in_page_rule() => {
@@ -709,12 +730,13 @@ impl PartialOrd for FontRelativeLength {
             (&Ex(ref one), &Ex(ref other)) => one.partial_cmp(other),
             (&Ch(ref one), &Ch(ref other)) => one.partial_cmp(other),
             (&Cap(ref one), &Cap(ref other)) => one.partial_cmp(other),
+            (&Ic(ref one), &Ic(ref other)) => one.partial_cmp(other),
             (&Rem(ref one), &Rem(ref other)) => one.partial_cmp(other),
             // See https://github.com/rust-lang/rust/issues/68867. rustc isn't
             // able to figure it own on its own so we help.
             _ => unsafe {
                 match *self {
-                    Em(..) | Ex(..) | Ch(..) | Cap(..) | Rem(..) => {},
+                    Em(..) | Ex(..) | Ch(..) | Cap(..) | Ic(..) | Rem(..) => {},
                 }
                 debug_unreachable!("Forgot an arm in partial_cmp?")
             },
@@ -732,6 +754,7 @@ impl Mul<CSSFloat> for FontRelativeLength {
             FontRelativeLength::Ex(v) => FontRelativeLength::Ex(v * scalar),
             FontRelativeLength::Ch(v) => FontRelativeLength::Ch(v * scalar),
             FontRelativeLength::Cap(v) => FontRelativeLength::Cap(v * scalar),
+            FontRelativeLength::Ic(v) => FontRelativeLength::Ic(v * scalar),
             FontRelativeLength::Rem(v) => FontRelativeLength::Rem(v * scalar),
         }
     }