diff --git a/float-pigment-css/src/group.rs b/float-pigment-css/src/group.rs
index 947bfc4..64286a8 100644
--- a/float-pigment-css/src/group.rs
+++ b/float-pigment-css/src/group.rs
@@ -685,14 +685,24 @@ impl StyleSheetGroup {
         path: &str,
         scope: Option<NonZeroUsize>,
     ) -> u16 {
+        self.append_from_resource_with_warnings(res, path, scope).0
+    }
+
+    /// Append a style sheet from the resource, returning its index and warnings like @import not found.
+    pub fn append_from_resource_with_warnings(
+        &mut self,
+        res: &StyleSheetResource,
+        path: &str,
+        scope: Option<NonZeroUsize>,
+    ) -> (u16, Vec<Warning>) {
         let path = drop_css_extension(path);
-        let (ss, _warnings) = res.link(path, scope);
+        let (ss, warnings) = res.link(path, scope);
         let ret = self.sheets.len();
         if Self::is_invalid_index(ret) {
             panic!("The number of stylesheets has reached the maximum limit.")
         }
         self.sheets.push(ss);
-        ret as u16
+        (ret as u16, warnings)
     }
 
     /// Replace a style sheet from the resource by its index.
diff --git a/float-pigment-css/src/sheet/font_face.rs b/float-pigment-css/src/sheet/font_face.rs
index f5189d2..88c3b83 100644
--- a/float-pigment-css/src/sheet/font_face.rs
+++ b/float-pigment-css/src/sheet/font_face.rs
@@ -15,6 +15,28 @@ pub struct FontFace {
     pub font_display: Option<FontDisplay>,
 }
 
+impl core::fmt::Display for FontFace {
+    fn fmt(&self, f: &mut core::fmt::Formatter<'_>) -> core::fmt::Result {
+        write!(f, "@font-face {{")?;
+        write!(f, " font-family: {};", self.font_family)?;
+        write!(f, " src: ")?;
+        for src in &self.src {
+            write!(f, "{}, ", src)?;
+        }
+        write!(f, ";")?;
+        if let Some(fs) = &self.font_style {
+            write!(f, " font-style: {};", fs)?;
+        }
+        if let Some(fw) = &self.font_weight {
+            write!(f, " font-weight: {};", fw)?;
+        }
+        if let Some(fd) = &self.font_display {
+            write!(f, " font-display: {};", fd)?;
+        }
+        write!(f, "}}")
+    }
+}
+
 #[allow(missing_docs)]
 #[derive(Debug, Clone, serde::Serialize, serde::Deserialize, PartialEq)]
 pub enum FontSrc {
@@ -22,6 +44,23 @@ pub enum FontSrc {
     Url(FontUrl),
 }
 
+impl core::fmt::Display for FontSrc {
+    fn fmt(&self, f: &mut core::fmt::Formatter<'_>) -> core::fmt::Result {
+        match self {
+            Self::Local(ff) => write!(f, r#"local("{}")"#, ff),
+            Self::Url(url) => {
+                write!(f, r#"url("{}")"#, url.url)?;
+                if let Some(formats) = &url.format {
+                    for format in formats {
+                        write!(f, r#" format("{}")"#, format)?;
+                    }
+                }
+                Ok(())
+            }
+        }
+    }
+}
+
 #[allow(missing_docs)]
 #[derive(Debug, Clone, serde::Serialize, serde::Deserialize, PartialEq, Eq)]
 pub struct FontUrl {
@@ -41,6 +80,18 @@ pub enum FontDisplay {
     Optional,
 }
 
+impl core::fmt::Display for FontDisplay {
+    fn fmt(&self, f: &mut core::fmt::Formatter<'_>) -> core::fmt::Result {
+        match self {
+            Self::Auto => write!(f, "auto"),
+            Self::Block => write!(f, "block"),
+            Self::Swap => write!(f, "swap"),
+            Self::Fallback => write!(f, "fallback"),
+            Self::Optional => write!(f, "optional"),
+        }
+    }
+}
+
 impl Default for FontFace {
     fn default() -> Self {
         Self {
diff --git a/float-pigment-css/src/sheet/keyframes.rs b/float-pigment-css/src/sheet/keyframes.rs
index 71ecb40..7f731c6 100644
--- a/float-pigment-css/src/sheet/keyframes.rs
+++ b/float-pigment-css/src/sheet/keyframes.rs
@@ -21,6 +21,16 @@ impl KeyFrames {
     }
 }
 
+impl core::fmt::Display for KeyFrames {
+    fn fmt(&self, f: &mut core::fmt::Formatter<'_>) -> core::fmt::Result {
+        write!(f, "@keyframes {} {{ ", self.ident)?;
+        for keyframe in &self.keyframes {
+            write!(f, "{} ", keyframe)?;
+        }
+        write!(f, "}}")
+    }
+}
+
 /// The percentage field in a keyframe item.
 #[repr(C)]
 #[derive(Clone, Debug, Serialize, Deserialize)]
@@ -61,3 +71,32 @@ impl KeyFrameRule {
         }
     }
 }
+
+impl core::fmt::Display for KeyFrameRule {
+    fn fmt(&self, f: &mut core::fmt::Formatter<'_>) -> core::fmt::Result {
+        write!(
+            f,
+            "{} {{ ",
+            self.keyframe
+                .iter()
+                .map(|x| {
+                    match x {
+                        KeyFrame::From => "from".to_owned(),
+                        KeyFrame::To => "to".to_owned(),
+                        KeyFrame::Ratio(ratio) => format!("{:.2}%", ratio * 100.),
+                    }
+                })
+                .collect::<Vec<_>>()
+                .join(", ")
+        )?;
+        for prop in &self.properties {
+            write!(
+                f,
+                "{}: {}; ",
+                prop.get_property_name(),
+                prop.get_property_value_string()
+            )?;
+        }
+        write!(f, "}}")
+    }
+}
diff --git a/float-pigment-css/src/sheet/mod.rs b/float-pigment-css/src/sheet/mod.rs
index fd7375f..d28846a 100644
--- a/float-pigment-css/src/sheet/mod.rs
+++ b/float-pigment-css/src/sheet/mod.rs
@@ -462,6 +462,21 @@ impl core::fmt::Debug for StyleSheet {
     }
 }
 
+impl core::fmt::Display for StyleSheet {
+    fn fmt(&self, f: &mut core::fmt::Formatter<'_>) -> core::fmt::Result {
+        for rule in self.rules.iter() {
+            write!(f, " {}", rule)?;
+        }
+        for font_face in self.font_face.iter() {
+            write!(f, " {}", font_face)?;
+        }
+        for keyframes in self.keyframes.iter() {
+            write!(f, " {}", keyframes)?;
+        }
+        Ok(())
+    }
+}
+
 impl StyleSheet {
     #[doc(hidden)]
     #[allow(clippy::should_implement_trait)]
diff --git a/float-pigment-css/src/sheet/rule.rs b/float-pigment-css/src/sheet/rule.rs
index 2b48339..d02d02e 100644
--- a/float-pigment-css/src/sheet/rule.rs
+++ b/float-pigment-css/src/sheet/rule.rs
@@ -5,7 +5,7 @@ use super::*;
 use crate::property::Property;
 
 /// A CSS property with some metadata.
-#[derive(Clone, Debug)]
+#[derive(Clone, Debug, PartialEq)]
 pub enum PropertyMeta {
     /// A single normal property, e.g. `font-size: 16px`.
     Normal {