diff --git a/crates/svg/src/lib.rs b/crates/svg/src/lib.rs
index 24cc584e..adddee0c 100644
--- a/crates/svg/src/lib.rs
+++ b/crates/svg/src/lib.rs
@@ -392,7 +392,17 @@ enum PaintRef {
     Current,
 }
 
-/// A CSS rule: selector (type, class, id; `*`) with its specificity and declarations.
+/// The most selectors (each entry of a comma list counts) the `<style>` sheets of a document may
+/// hold (#2140).
+const MAX_CSS_SELECTORS: usize = 20_000;
+/// The most declarations the `<style>` sheets of a document may hold, each block counted once.
+const MAX_CSS_DECLARATIONS: usize = 50_000;
+
+/// The most matching work (candidate rules checked plus declarations applied) the elements of a
+/// document may cost, `<use>` copies included: a bound on elements x rules x declarations.
+const MAX_CSS_WORK: usize = 50_000_000;
+
+/// A CSS rule: selector (type, class, id; `*`) with its specificity and declaration block.
 #[derive(Clone, Debug)]
 struct Rule {
     tag: Option<String>,
@@ -400,7 +410,39 @@ struct Rule {
     id: Option<String>,
     specificity: u32,
     order: usize,
-    decls: Vec<(String, String)>,
+    /// Index into [`StyleSheet::blocks`]: every selector of a comma list shares one block.
+    block: usize,
+}
+
+/// The rules of a document's `<style>` sheets, indexed by the part of the selector an element
+/// must have (id, else first class, else type) so an element only checks rules that can match.
+#[derive(Default, Debug)]
+struct StyleSheet {
+    rules: Vec<Rule>,
+    blocks: Vec<Vec<(String, String)>>,
+    /// Declarations over all blocks.
+    decls: usize,
+    by_id: HashMap<String, Vec<usize>>,
+    by_class: HashMap<String, Vec<usize>>,
+    by_tag: HashMap<String, Vec<usize>>,
+    universal: Vec<usize>,
+}
+
+impl StyleSheet {
+    fn index(&mut self) {
+        for (i, r) in self.rules.iter().enumerate() {
+            let bucket = if let Some(id) = &r.id {
+                self.by_id.entry(id.clone()).or_default()
+            } else if let Some(c) = r.classes.first() {
+                self.by_class.entry(c.clone()).or_default()
+            } else if let Some(t) = &r.tag {
+                self.by_tag.entry(t.clone()).or_default()
+            } else {
+                &mut self.universal
+            };
+            bucket.push(i);
+        }
+    }
 }
 
 fn parse_decls(s: &str) -> Vec<(String, String)> {
@@ -428,7 +470,9 @@ fn strip_comments(s: &str) -> String {
     out
 }
 
-fn parse_css(css: &str, rules: &mut Vec<Rule>) {
+/// Add the rules of one `<style>` sheet. Errs once the sheets exceed [`MAX_CSS_SELECTORS`] or
+/// [`MAX_CSS_DECLARATIONS`] (#2140).
+fn parse_css(css: &str, sheet: &mut StyleSheet) -> Result<(), Error> {
     let css = strip_comments(css);
     let mut rest = css.as_str();
     while let Some(open) = rest.find('{') {
@@ -439,13 +483,28 @@ fn parse_css(css: &str, rules: &mut Vec<Rule>) {
         if sel.starts_with('@') {
             continue;
         }
-        let decls = parse_decls(body);
+        let mut decls = parse_decls(body);
+        sheet.decls = sheet.decls.saturating_add(decls.len());
+        if sheet.decls > MAX_CSS_DECLARATIONS {
+            return Err(Error::Xml(format!("its <style> sheets hold more than {MAX_CSS_DECLARATIONS} declarations")));
+        }
+        // Stored with the rule's first accepted selector: a rule of unsupported selectors only
+        // (`a b{}`, repeated) adds no block.
+        let mut block = None;
         for one in sel.split(',') {
             let one = one.trim();
             // Only simple selectors (no combinators / pseudo-classes / attributes).
             if one.is_empty() || one.contains([' ', '>', '+', '~', ':', '[']) {
                 continue;
             }
+            if sheet.rules.len() >= MAX_CSS_SELECTORS {
+                return Err(Error::Xml(format!("its <style> sheets hold more than {MAX_CSS_SELECTORS} selectors")));
+            }
+            let block = *block.get_or_insert_with(|| {
+                let i = sheet.blocks.len();
+                sheet.blocks.push(std::mem::take(&mut decls));
+                i
+            });
             let mut tag = None;
             let mut classes = vec![];
             let mut id = None;
@@ -474,11 +533,14 @@ fn parse_css(css: &str, rules: &mut Vec<Rule>) {
                 }
             }
             flush(kind, &mut cur, &mut tag, &mut classes, &mut id);
-            let specificity = id.is_some() as u32 * 100 + classes.len() as u32 * 10 + tag.is_some() as u32;
-            let order = rules.len();
-            rules.push(Rule { tag, classes, id, specificity, order, decls: decls.clone() });
+            let specificity = (id.is_some() as u32 * 100)
+                .saturating_add(u32::try_from(classes.len()).unwrap_or(u32::MAX).saturating_mul(10))
+                .saturating_add(tag.is_some() as u32);
+            let order = sheet.rules.len();
+            sheet.rules.push(Rule { tag, classes, id, specificity, order, block });
         }
     }
+    Ok(())
 }
 
 const PROPS: &[&str] = &[
@@ -505,7 +567,9 @@ const PROPS: &[&str] = &[
 
 struct Parser<'a, 'i> {
     ids: HashMap<String, roxmltree::Node<'a, 'i>>,
-    rules: Vec<Rule>,
+    css: StyleSheet,
+    /// CSS matching work done so far; past [`MAX_CSS_WORK`] rules stop applying and the import errs.
+    css_work: std::cell::Cell<usize>,
     vw: f64,
     vh: f64,
     skipped: Vec<String>,
@@ -531,12 +595,50 @@ impl<'a, 'i> Parser<'a, 'i> {
                 out.insert(a.name().to_string(), a.value().to_string());
             }
         }
+        self.apply_rules(n, &mut out);
+        if let Some(st) = n.attribute("style") {
+            for (k, v) in parse_decls(st) {
+                out.insert(k, v);
+            }
+        }
+        out
+    }
+
+    /// Charge `n` units of CSS work; false once the budget is spent.
+    fn charge_css(&self, n: usize) -> bool {
+        let w = self.css_work.get().saturating_add(n);
+        self.css_work.set(w);
+        w <= MAX_CSS_WORK
+    }
+
+    /// Apply the matching `<style>` rules to `out`, by specificity then source order.
+    fn apply_rules(&self, n: roxmltree::Node, out: &mut HashMap<String, String>) {
+        let css = &self.css;
+        if css.rules.is_empty() {
+            return;
+        }
+        // Splitting and sorting the class list is work too: charge its length first.
+        let class = n.attribute("class").unwrap_or_default();
+        if !self.charge_css(class.len()) {
+            return;
+        }
         let tag = n.tag_name().name();
-        let classes: Vec<&str> = n.attribute("class").map(|c| c.split_whitespace().collect()).unwrap_or_default();
+        let mut classes: Vec<&str> = class.split_whitespace().collect();
+        classes.sort_unstable();
+        classes.dedup();
         let id = n.attribute("id");
-        let mut matching: Vec<&Rule> = self
-            .rules
+        // Each rule sits in exactly one bucket and the classes are distinct: no rule twice.
+        let mut buckets: Vec<&Vec<usize>> = vec![&css.universal];
+        buckets.extend(id.and_then(|i| css.by_id.get(i)));
+        buckets.extend(classes.iter().filter_map(|c| css.by_class.get(*c)));
+        buckets.extend(css.by_tag.get(tag));
+        if !self.charge_css(buckets.iter().map(|b| b.len()).fold(0, usize::saturating_add)) {
+            return;
+        }
+        let mut matching: Vec<&Rule> = buckets
             .iter()
+            .flat_map(|b| b.iter())
+            .filter_map(|&i| css.rules.get(i))
             .filter(|r| {
                 r.tag.as_deref().is_none_or(|t| t == tag)
                     && r.id.as_deref().is_none_or(|i| Some(i) == id)
@@ -544,17 +646,13 @@ impl<'a, 'i> Parser<'a, 'i> {
             })
             .collect();
         matching.sort_by_key(|r| (r.specificity, r.order));
-        for r in matching {
-            for (k, v) in &r.decls {
-                out.insert(k.clone(), v.clone());
-            }
+        let blocks: Vec<&Vec<(String, String)>> = matching.iter().filter_map(|r| css.blocks.get(r.block)).collect();
+        if !self.charge_css(blocks.iter().map(|b| b.len()).fold(0, usize::saturating_add)) {
+            return;
         }
-        if let Some(st) = n.attribute("style") {
-            for (k, v) in parse_decls(st) {
-                out.insert(k, v);
-            }
+        for (k, v) in blocks.into_iter().flatten() {
+            out.insert(k.clone(), v.clone());
         }
-        out
     }
 
     fn length(&self, s: &str, axis: char, font: f64) -> Option<f64> {
@@ -1147,20 +1245,31 @@ pub fn parse(bytes: &[u8]) -> Result<Doc, Error> {
         return Err(Error::NotSvg);
     }
     let mut ids = HashMap::new();
-    let mut rules = vec![];
+    let mut css = StyleSheet::default();
     for n in root.descendants().filter(|n| n.is_element()) {
         if let Some(id) = n.attribute("id") {
             ids.entry(id.to_string()).or_insert(n);
         }
         if n.tag_name().name() == "style" {
-            let css: String = n.children().filter_map(|c| c.text()).collect();
-            parse_css(&css, &mut rules);
+            let text: String = n.children().filter_map(|c| c.text()).collect();
+            parse_css(&text, &mut css)?;
         }
     }
+    css.index();
     let vb = viewbox(root);
     // Room for generous reuse, and always for every element of the file itself.
     let budget = MAX_EXPANDED_ELEMENTS.max(root.descendants().filter(|n| n.is_element()).count().saturating_mul(2));
-    let mut p = Parser { ids, rules, vw: vb.map_or(300.0, |v| v.1), vh: vb.map_or(150.0, |v| v.2), skipped: vec![], work: 0, budget, using: vec![] };
+    let mut p = Parser {
+        ids,
+        css,
+        css_work: std::cell::Cell::new(0),
+        vw: vb.map_or(300.0, |v| v.1),
+        vh: vb.map_or(150.0, |v| v.2),
+        skipped: vec![],
+        work: 0,
+        budget,
+        using: vec![],
+    };
     let st = Style::default();
     let pct_or = |name: &str, axis: char, def: f64| -> f64 {
         match root.attribute(name) {
@@ -1200,6 +1309,10 @@ pub fn parse(bytes: &[u8]) -> Result<Doc, Error> {
         // Never a silently partial drawing.
         return Err(Error::Xml(format!("its <use> references expand to more than {} elements", p.budget)));
     }
+    if p.css_work.get() > MAX_CSS_WORK {
+        // Never a silently unstyled drawing.
+        return Err(Error::Xml(format!("matching its <style> rules takes more than {MAX_CSS_WORK} steps")));
+    }
     let transform = viewbox_transform(root, dw, dh);
     Ok(Doc {
         width: dw,
diff --git a/crates/svg/src/tests.rs b/crates/svg/src/tests.rs
index ee13a79f..ff6504a5 100644
--- a/crates/svg/src/tests.rs
+++ b/crates/svg/src/tests.rs
@@ -296,3 +296,98 @@ fn a_symbol_used_a_thousand_times_still_imports() {
     let doc = parse(svg.as_bytes()).unwrap();
     assert_eq!(depth_and_shapes(&doc.root).1, 3000);
 }
+
+/// An SVG with one `<style>` sheet and one rectangle per given class list.
+fn styled(css: &str, rects: &[&str]) -> Vec<u8> {
+    let rects: String = rects.iter().map(|attrs| format!(r#"<rect width="1" height="1" {attrs}/>"#)).collect();
+    format!(r#"<svg xmlns="http://www.w3.org/2000/svg" width="10" height="10"><style>{css}</style>{rects}</svg>"#).into_bytes()
+}
+
+#[test]
+fn comma_selector_lists_share_one_declaration_block() {
+    // Before #2140 every selector of a comma list got its own copy of the declaration block, so
+    // `a,a,a…{…}` stored selectors × declarations strings; one selector past the cap is refused.
+    let sels = vec!["rect"; MAX_CSS_SELECTORS + 1].join(",");
+    let decls: String = (0..10).map(|i| format!("p{i}:v;")).collect();
+    let svg = styled(&format!("{sels}{{fill:red;{decls}}}"), &[""]);
+    let e = parse(&svg).unwrap_err();
+    assert!(e.to_string().contains("selectors"), "{e}");
+}
+
+#[test]
+fn a_style_sheet_with_too_many_declarations_errors() {
+    let decls: String = (0..=MAX_CSS_DECLARATIONS).map(|i| format!("p{i}:v;")).collect();
+    let svg = styled(&format!("rect{{{decls}}}"), &[""]);
+    let e = parse(&svg).unwrap_err();
+    assert!(e.to_string().contains("declarations"), "{e}");
+}
+
+#[test]
+fn selector_lists_keep_the_css_cascade() {
+    let green = Some(Paint::Color([0.0, 1.0, 0.0]));
+    let blue = Some(Paint::Color([0.0, 0.0, 1.0]));
+    let red = Some(Paint::Color([1.0, 0.0, 0.0]));
+    let css = "rect, .b, #c { fill: #00ff00 } .b, .d { fill: #0000ff; stroke: #000000; stroke-width: 3 } rect.e, #c { fill: #ff0000 }";
+    let svg = styled(css, &["", r#"class="b""#, r#"id="c" class="b""#, r#"class="b" style="fill:#00ff00""#, r##"class="e" fill="#0000ff""##, r#"class="d""#]);
+    let doc = parse(&svg).unwrap();
+    let shapes = doc.flatten();
+    let fills: Vec<_> = shapes.iter().map(|s| s.2.fill.clone()).collect();
+    // Type selector; class beats type; id beats class; inline style beats every rule; a later,
+    // more specific rule beats the presentation attribute.
+    assert_eq!(fills, vec![green.clone(), blue.clone(), red.clone(), green, red, blue]);
+    assert!(shapes[0].2.stroke.is_none());
+    // The second list's block reaches both of its selectors.
+    for i in [1, 5] {
+        assert_eq!(shapes[i].2.stroke.as_ref().map(|s| s.width), Some(3.0), "shape {i}");
+    }
+}
+
+#[test]
+fn a_comma_list_stores_its_declarations_once() {
+    let sels = vec!["rect"; 1000].join(",");
+    let decls: String = (0..50).map(|i| format!("p{i}:v;")).collect();
+    let mut sheet = StyleSheet::default();
+    parse_css(&format!("{sels}{{{decls}}}"), &mut sheet).unwrap();
+    assert_eq!(sheet.rules.len(), 1000);
+    assert_eq!(sheet.blocks.len(), 1);
+    assert_eq!(sheet.decls, 50);
+}
+
+#[test]
+fn rules_of_unsupported_selectors_store_no_blocks() {
+    let mut sheet = StyleSheet::default();
+    parse_css(&"a b{}".repeat(100_000), &mut sheet).unwrap();
+    parse_css("a b{fill:red} a>b, rect{fill:blue} c:hover, d{fill:red}", &mut sheet).unwrap();
+    assert_eq!(sheet.rules.len(), 2);
+    assert_eq!(sheet.blocks.len(), 2);
+    assert!(sheet.rules.iter().all(|r| sheet.blocks.get(r.block).is_some_and(|b| b.len() == 1)));
+}
+
+#[test]
+fn huge_class_lists_count_against_the_matching_budget() {
+    // One rule, but a 1 MB class attribute re-split on each of 100 `<use>` copies (#2140).
+    let class = "a ".repeat(500_000);
+    let uses: String = (0..100).map(|i| format!(r##"<use href="#r" x="{i}"/>"##)).collect();
+    let svg = |uses: &str| {
+        format!(
+            r##"<svg xmlns="http://www.w3.org/2000/svg" width="10" height="10"><style>.z{{fill:red}}</style><defs><rect id="r" width="1" height="1" class="{class}"/></defs>{uses}</svg>"##
+        )
+    };
+    let e = parse(svg(&uses).as_bytes()).unwrap_err();
+    assert!(e.to_string().contains("<style> rules"), "{e}");
+    let few: String = uses.split_inclusive("/>").take(10).collect();
+    assert!(parse(svg(&few).as_bytes()).is_ok());
+}
+
+#[test]
+fn matching_work_past_the_budget_errors_without_doing_it() {
+    // Within both sheet caps, yet one rectangle would apply 10 000 x 5 001 declarations.
+    let sels = vec!["rect"; 10_000].join(",");
+    let decls: String = (0..5_001).map(|i| format!("p{i}:v;")).collect();
+    let svg = styled(&format!("{sels}{{{decls}}}"), &[""]);
+    let e = parse(&svg).unwrap_err();
+    assert!(e.to_string().contains("<style> rules"), "{e}");
+    // The same sheet styles an element it does not match at no cost.
+    let ok = styled(&format!("{}{{{decls}}}", vec!["circle"; 10_000].join(",")), &[""]);
+    assert!(parse(&ok).is_ok());
+}