diff --git a/components/script/dom/element.rs b/components/script/dom/element.rs
index 311991804e9..4db03cfb3b3 100644
--- a/components/script/dom/element.rs
+++ b/components/script/dom/element.rs
@@ -4481,7 +4481,9 @@ impl SelectorsElement for SelectorWrapper<'_> {
             // a string containing commas (separating each language tag in
             // a list) but the pseudo-class instead should be parsing and
             // storing separate <ident> or <string>s for each language tag.
-            NonTSPseudoClass::Lang(ref lang) => extended_filtering(&self.get_lang(), lang),
+            NonTSPseudoClass::Lang(ref lang) => {
+                extended_filtering(&self.upcast::<Node>().get_lang().unwrap_or_default(), lang)
+            },
 
             NonTSPseudoClass::ReadOnly => {
                 !Element::state(self).contains(NonTSPseudoClass::ReadWrite.state_flag())
@@ -4821,24 +4823,7 @@ impl Element {
         }
     }
 
-    // https://html.spec.whatwg.org/multipage/#language
-    pub(crate) fn get_lang(&self) -> String {
-        self.upcast::<Node>()
-            .inclusive_ancestors(ShadowIncluding::Yes)
-            .filter_map(|node| {
-                node.downcast::<Element>().and_then(|el| {
-                    el.get_attribute(&ns!(xml), &local_name!("lang"))
-                        .or_else(|| el.get_attribute(&ns!(), &local_name!("lang")))
-                        .map(|attr| String::from(attr.Value()))
-                })
-                // TODO: Check meta tags for a pragma-set default language
-                // TODO: Check HTTP Content-Language header
-            })
-            .next()
-            .unwrap_or(String::new())
-    }
-
-    pub(crate) fn state(&self) -> ElementState {
+    pub fn state(&self) -> ElementState {
         self.state.get()
     }
 
diff --git a/components/script/dom/node.rs b/components/script/dom/node.rs
index 9c14a840849..ab814bc9108 100644
--- a/components/script/dom/node.rs
+++ b/components/script/dom/node.rs
@@ -48,7 +48,7 @@ use style::properties::ComputedValues;
 use style::selector_parser::{SelectorImpl, SelectorParser};
 use style::stylesheets::{Stylesheet, UrlExtraData};
 use uuid::Uuid;
-use xml5ever::serialize as xml_serialize;
+use xml5ever::{local_name, serialize as xml_serialize};
 
 use crate::conversions::Convert;
 use crate::document_loader::DocumentLoader;
@@ -1470,6 +1470,21 @@ impl Node {
             .map(|data| data.element_data.borrow().styles.primary().clone())
     }
 
+    /// <https://html.spec.whatwg.org/multipage/#language>
+    pub(crate) fn get_lang(&self) -> Option<String> {
+        self.inclusive_ancestors(ShadowIncluding::Yes)
+            .filter_map(|node| {
+                node.downcast::<Element>().and_then(|el| {
+                    el.get_attribute(&ns!(xml), &local_name!("lang"))
+                        .or_else(|| el.get_attribute(&ns!(), &local_name!("lang")))
+                        .map(|attr| String::from(attr.Value()))
+                })
+                // TODO: Check meta tags for a pragma-set default language
+                // TODO: Check HTTP Content-Language header
+            })
+            .next()
+    }
+
     /// <https://dom.spec.whatwg.org/#assign-slotables-for-a-tree>
     pub(crate) fn assign_slottables_for_a_tree(&self) {
         // NOTE: This method traverses all descendants of the node and is potentially very
diff --git a/components/script/dom/xpathresult.rs b/components/script/dom/xpathresult.rs
index f4a9ae7a31b..890e559ba97 100644
--- a/components/script/dom/xpathresult.rs
+++ b/components/script/dom/xpathresult.rs
@@ -54,7 +54,7 @@ impl TryFrom<u16> for XPathResultType {
     }
 }
 
-#[derive(JSTraceable, MallocSizeOf)]
+#[derive(Debug, JSTraceable, MallocSizeOf)]
 pub(crate) enum XPathResultValue {
     Boolean(bool),
     /// A IEEE-754 double-precision floating point number
diff --git a/components/script/xpath/eval.rs b/components/script/xpath/eval.rs
index 75d7ac5849c..4fd70ec1c97 100644
--- a/components/script/xpath/eval.rs
+++ b/components/script/xpath/eval.rs
@@ -83,6 +83,22 @@ where
     }
 }
 
+impl<T> Evaluatable for Option<T>
+where
+    T: Evaluatable,
+{
+    fn evaluate(&self, context: &EvaluationCtx) -> Result<Value, Error> {
+        match self {
+            Some(expr) => expr.evaluate(context),
+            None => Ok(Value::Nodeset(vec![])),
+        }
+    }
+
+    fn is_primitive(&self) -> bool {
+        self.as_ref().is_some_and(|t| T::is_primitive(t))
+    }
+}
+
 impl Evaluatable for Expr {
     fn evaluate(&self, context: &EvaluationCtx) -> Result<Value, Error> {
         match self {
diff --git a/components/script/xpath/eval_function.rs b/components/script/xpath/eval_function.rs
index caf0782c07b..53c14944474 100644
--- a/components/script/xpath/eval_function.rs
+++ b/components/script/xpath/eval_function.rs
@@ -2,12 +2,15 @@
  * License, v. 2.0. If a copy of the MPL was not distributed with this
  * file, You can obtain one at https://mozilla.org/MPL/2.0/. */
 
+use style::Atom;
+
 use super::Value;
 use super::context::EvaluationCtx;
 use super::eval::{Error, Evaluatable, try_extract_nodeset};
 use super::parser::CoreFunction;
 use crate::dom::bindings::codegen::Bindings::NodeBinding::NodeMethods;
 use crate::dom::bindings::inheritance::{Castable, NodeTypeId};
+use crate::dom::bindings::root::DomRoot;
 use crate::dom::element::Element;
 use crate::dom::node::Node;
 
@@ -101,6 +104,31 @@ pub(crate) fn normalize_space(s: &str) -> String {
     result
 }
 
+/// <https://www.w3.org/TR/1999/REC-xpath-19991116/#function-lang>
+fn lang_matches(context_lang: Option<&str>, target_lang: &str) -> bool {
+    let Some(context_lang) = context_lang else {
+        return false;
+    };
+
+    let context_lower = context_lang.to_ascii_lowercase();
+    let target_lower = target_lang.to_ascii_lowercase();
+
+    if context_lower == target_lower {
+        return true;
+    }
+
+    // Check if context is target with additional suffix
+    if context_lower.starts_with(&target_lower) {
+        // Make sure the next character is a hyphen to avoid matching
+        // e.g. "england" when target is "en"
+        if let Some(next_char) = context_lower.chars().nth(target_lower.len()) {
+            return next_char == '-';
+        }
+    }
+
+    false
+}
+
 impl Evaluatable for CoreFunction {
     fn evaluate(&self, context: &EvaluationCtx) -> Result<Value, Error> {
         match self {
@@ -131,7 +159,20 @@ impl Evaluatable for CoreFunction {
                     .collect();
                 Ok(Value::String(strings?.join("")))
             },
-            CoreFunction::Id(_expr) => todo!(),
+            CoreFunction::Id(expr) => {
+                let args_str = expr.evaluate(context)?.string();
+                let args_normalized = normalize_space(&args_str);
+                let args = args_normalized.split(' ');
+
+                let document = context.context_node.owner_doc();
+                let mut result = Vec::new();
+                for arg in args {
+                    for element in document.get_elements_with_id(&Atom::from(arg)).iter() {
+                        result.push(DomRoot::from_ref(element.upcast::<Node>()));
+                    }
+                }
+                Ok(Value::Nodeset(result))
+            },
             CoreFunction::LocalName(expr_opt) => {
                 let node = match expr_opt {
                     Some(expr) => expr
@@ -256,7 +297,11 @@ impl Evaluatable for CoreFunction {
             CoreFunction::Not(expr) => Ok(Value::Boolean(!expr.evaluate(context)?.boolean())),
             CoreFunction::True => Ok(Value::Boolean(true)),
             CoreFunction::False => Ok(Value::Boolean(false)),
-            CoreFunction::Lang(_) => Ok(Value::Nodeset(vec![])), // Not commonly used in the DOM, short-circuit it
+            CoreFunction::Lang(expr) => {
+                let context_lang = context.context_node.get_lang();
+                let lang = expr.evaluate(context)?.string();
+                Ok(Value::Boolean(lang_matches(context_lang.as_deref(), &lang)))
+            },
         }
     }
 
@@ -319,7 +364,7 @@ impl Evaluatable for CoreFunction {
 }
 #[cfg(test)]
 mod tests {
-    use super::{substring, substring_after, substring_before};
+    use super::{lang_matches, substring, substring_after, substring_before};
 
     #[test]
     fn test_substring_before() {
@@ -354,4 +399,18 @@ mod tests {
         assert_eq!(substring("hello", 0, Some(0)), "");
         assert_eq!(substring("hello", 0, Some(-5)), "");
     }
+
+    #[test]
+    fn test_lang_matches() {
+        assert!(lang_matches(Some("en"), "en"));
+        assert!(lang_matches(Some("EN"), "en"));
+        assert!(lang_matches(Some("en"), "EN"));
+        assert!(lang_matches(Some("en-US"), "en"));
+        assert!(lang_matches(Some("en-GB"), "en"));
+
+        assert!(!lang_matches(Some("eng"), "en"));
+        assert!(!lang_matches(Some("fr"), "en"));
+        assert!(!lang_matches(Some("fr-en"), "en"));
+        assert!(!lang_matches(None, "en"));
+    }
 }
diff --git a/components/script/xpath/eval_value.rs b/components/script/xpath/eval_value.rs
index de6c13e3454..66f1b92c6d4 100644
--- a/components/script/xpath/eval_value.rs
+++ b/components/script/xpath/eval_value.rs
@@ -8,7 +8,6 @@ use std::{fmt, string};
 
 use crate::dom::bindings::codegen::Bindings::NodeBinding::Node_Binding::NodeMethods;
 use crate::dom::bindings::root::DomRoot;
-use crate::dom::bindings::utils::AsVoidPtr;
 use crate::dom::node::Node;
 
 /// The primary types of values that an XPath expression returns as a result.
@@ -216,7 +215,7 @@ impl NodesetHelpers for Vec<DomRoot<Node>> {
     }
     fn document_order(&self) -> Vec<DomRoot<Node>> {
         let mut nodes: Vec<DomRoot<Node>> = self.clone();
-        if nodes.len() == 1 {
+        if nodes.len() <= 1 {
             return nodes;
         }
 
@@ -233,10 +232,13 @@ impl NodesetHelpers for Vec<DomRoot<Node>> {
         nodes
     }
     fn document_order_unique(&self) -> Vec<DomRoot<Node>> {
-        let mut nodes: Vec<DomRoot<Node>> = self.document_order();
+        let mut seen = HashSet::new();
+        let unique_nodes: Vec<DomRoot<Node>> = self
+            .iter()
+            .filter(|node| seen.insert(node.to_opaque()))
+            .cloned()
+            .collect();
 
-        nodes.dedup_by_key(|n| n.as_void_ptr());
-
-        nodes
+        unique_nodes.document_order()
     }
 }
diff --git a/components/script/xpath/parser.rs b/components/script/xpath/parser.rs
index b1a4bfcc42d..2516dcc5478 100644
--- a/components/script/xpath/parser.rs
+++ b/components/script/xpath/parser.rs
@@ -542,7 +542,7 @@ fn relative_path_expr(input: &str) -> IResult<&str, Expr> {
     let (input, first) = step_expr(input)?;
     let (input, steps) = many0(pair(
         // ("/" | "//")
-        ws(alt((value(false, char('/')), value(true, tag("//"))))),
+        ws(alt((value(true, tag("//")), value(false, char('/'))))),
         step_expr,
     ))(input)?;
 
@@ -592,12 +592,7 @@ fn axis_step(input: &str) -> IResult<&str, AxisStep> {
 }
 
 fn forward_step(input: &str) -> IResult<&str, (Axis, NodeTest)> {
-    alt((
-        // ForwardAxis NodeTest
-        pair(forward_axis, node_test),
-        // AbbrevForwardStep
-        abbrev_forward_step,
-    ))(input)
+    alt((pair(forward_axis, node_test), abbrev_forward_step))(input)
 }
 
 fn forward_axis(input: &str) -> IResult<&str, Axis> {
@@ -704,6 +699,7 @@ fn filter_expr(input: &str) -> IResult<&str, FilterExpr> {
 
 fn predicate_list(input: &str) -> IResult<&str, PredicateListExpr> {
     let (input, predicates) = many0(predicate)(input)?;
+
     Ok((input, PredicateListExpr { predicates }))
 }
 
@@ -1195,6 +1191,118 @@ mod tests {
                     ],
                 }),
             ),
+            (
+                "//mu[@xml:id=\"id1\"]//rho[@title][@xml:lang=\"en-GB\"]",
+                Expr::Path(PathExpr {
+                    is_absolute: true,
+                    is_descendant: true,
+                    steps: vec![
+                        StepExpr::Axis(AxisStep {
+                            axis: Axis::Child,
+                            node_test: NodeTest::Name(QName {
+                                prefix: None,
+                                local_part: "mu".to_string(),
+                            }),
+                            predicates: PredicateListExpr {
+                                predicates: vec![PredicateExpr {
+                                    expr: Expr::Equality(
+                                        Box::new(Expr::Path(PathExpr {
+                                            is_absolute: false,
+                                            is_descendant: false,
+                                            steps: vec![StepExpr::Axis(AxisStep {
+                                                axis: Axis::Attribute,
+                                                node_test: NodeTest::Name(QName {
+                                                    prefix: Some("xml".to_string()),
+                                                    local_part: "id".to_string(),
+                                                }),
+                                                predicates: PredicateListExpr {
+                                                    predicates: vec![],
+                                                },
+                                            })],
+                                        })),
+                                        EqualityOp::Eq,
+                                        Box::new(Expr::Path(PathExpr {
+                                            is_absolute: false,
+                                            is_descendant: false,
+                                            steps: vec![StepExpr::Filter(FilterExpr {
+                                                primary: PrimaryExpr::Literal(Literal::String(
+                                                    "id1".to_string(),
+                                                )),
+                                                predicates: PredicateListExpr {
+                                                    predicates: vec![],
+                                                },
+                                            })],
+                                        })),
+                                    ),
+                                }],
+                            },
+                        }),
+                        StepExpr::Axis(AxisStep {
+                            axis: Axis::DescendantOrSelf, // Represents the second '//'
+                            node_test: NodeTest::Kind(KindTest::Node),
+                            predicates: PredicateListExpr { predicates: vec![] },
+                        }),
+                        StepExpr::Axis(AxisStep {
+                            axis: Axis::Child,
+                            node_test: NodeTest::Name(QName {
+                                prefix: None,
+                                local_part: "rho".to_string(),
+                            }),
+                            predicates: PredicateListExpr {
+                                predicates: vec![
+                                    PredicateExpr {
+                                        expr: Expr::Path(PathExpr {
+                                            is_absolute: false,
+                                            is_descendant: false,
+                                            steps: vec![StepExpr::Axis(AxisStep {
+                                                axis: Axis::Attribute,
+                                                node_test: NodeTest::Name(QName {
+                                                    prefix: None,
+                                                    local_part: "title".to_string(),
+                                                }),
+                                                predicates: PredicateListExpr {
+                                                    predicates: vec![],
+                                                },
+                                            })],
+                                        }),
+                                    },
+                                    PredicateExpr {
+                                        expr: Expr::Equality(
+                                            Box::new(Expr::Path(PathExpr {
+                                                is_absolute: false,
+                                                is_descendant: false,
+                                                steps: vec![StepExpr::Axis(AxisStep {
+                                                    axis: Axis::Attribute,
+                                                    node_test: NodeTest::Name(QName {
+                                                        prefix: Some("xml".to_string()),
+                                                        local_part: "lang".to_string(),
+                                                    }),
+                                                    predicates: PredicateListExpr {
+                                                        predicates: vec![],
+                                                    },
+                                                })],
+                                            })),
+                                            EqualityOp::Eq,
+                                            Box::new(Expr::Path(PathExpr {
+                                                is_absolute: false,
+                                                is_descendant: false,
+                                                steps: vec![StepExpr::Filter(FilterExpr {
+                                                    primary: PrimaryExpr::Literal(Literal::String(
+                                                        "en-GB".to_string(),
+                                                    )),
+                                                    predicates: PredicateListExpr {
+                                                        predicates: vec![],
+                                                    },
+                                                })],
+                                            })),
+                                        ),
+                                    },
+                                ],
+                            },
+                        }),
+                    ],
+                }),
+            ),
         ];
 
         for (input, expected) in cases {
diff --git a/tests/wpt/meta/MANIFEST.json b/tests/wpt/meta/MANIFEST.json
index 20dccf72454..400ef8b2ccf 100644
--- a/tests/wpt/meta/MANIFEST.json
+++ b/tests/wpt/meta/MANIFEST.json
@@ -631177,6 +631177,13 @@
       {}
      ]
     ],
+    "fn-id.html": [
+     "617ff7216d84d56f8cef3f8ca08e902dd67f487d",
+     [
+      null,
+      {}
+     ]
+    ],
     "fn-lang.html": [
      "1fbd0a2ee4d419275c6d78f54e3425135cf838a1",
      [
diff --git a/tests/wpt/meta/domxpath/fn-lang.html.ini b/tests/wpt/meta/domxpath/fn-lang.html.ini
deleted file mode 100644
index 31754463dcc..00000000000
--- a/tests/wpt/meta/domxpath/fn-lang.html.ini
+++ /dev/null
@@ -1,21 +0,0 @@
-[fn-lang.html]
-  [lang("en"): <root><match lang="en"></match></root>]
-    expected: FAIL
-
-  [lang("en"): <root><match lang="EN"></match></root>]
-    expected: FAIL
-
-  [lang("en"): <root><match lang="en-us"></match></root>]
-    expected: FAIL
-
-  [lang("en"): <root><unmatch></unmatch></root>]
-    expected: FAIL
-
-  [lang("ja"): <root lang="ja"><match></match></root>]
-    expected: FAIL
-
-  [lang("ja"): <root lang="ja-jp"><unmatch lang="ja_JP"></unmatch></root>]
-    expected: FAIL
-
-  [lang("ko"): <root><unmatch lang="Ko"></unmatch></root>]
-    expected: FAIL
diff --git a/tests/wpt/meta/domxpath/node-sets.html.ini b/tests/wpt/meta/domxpath/node-sets.html.ini
deleted file mode 100644
index 856f22dd655..00000000000
--- a/tests/wpt/meta/domxpath/node-sets.html.ini
+++ /dev/null
@@ -1,3 +0,0 @@
-[node-sets.html]
-  [| operator should evaluate both sides of expressions with the same context node]
-    expected: FAIL
diff --git a/tests/wpt/tests/domxpath/fn-id.html b/tests/wpt/tests/domxpath/fn-id.html
new file mode 100644
index 00000000000..617ff7216d8
--- /dev/null
+++ b/tests/wpt/tests/domxpath/fn-id.html
@@ -0,0 +1,47 @@
+<!DOCTYPE html>
+<link rel="help" href="https://www.w3.org/TR/1999/REC-xpath-19991116/#function-id">
+<body>
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<script>
+// Test the id() function with various scenarios
+function testIdFunction(expression, xmlString, expectedIds) {
+  let doc = (new DOMParser()).parseFromString(xmlString, 'text/xml');
+  test(() => {
+    let result = doc.evaluate(expression, doc.documentElement, null, XPathResult.ORDERED_NODE_SNAPSHOT_TYPE, null);
+    assert_equals(result.resultType, XPathResult.ORDERED_NODE_SNAPSHOT_TYPE);
+    let actualIds = [];
+    for (let i = 0; i < result.snapshotLength; i++) {
+      actualIds.push(result.snapshotItem(i).getAttribute('id'));
+    }
+    actualIds.sort();
+    expectedIds.sort();
+    assert_array_equals(actualIds, expectedIds, `Expected IDs ${expectedIds}, got ${actualIds}`);
+  }, `${expression}: ${doc.documentElement.outerHTML}`);
+}
+
+// Test single ID
+testIdFunction('id("test1")', '<root><div id="test1">Match</div></root>', ['test1']);
+
+// Test multiple IDs in space-separated string
+testIdFunction('id("test1 test2")', '<root><div id="test1">First</div><div id="test2">Second</div></root>', ['test1', 'test2']);
+
+// Test non-existent ID
+testIdFunction('id("nonexistent")', '<root><div id="test1">No match</div></root>', []);
+
+// Test mixed case IDs (should be case-sensitive)
+testIdFunction('id("Test1")', '<root><div id="test1">No match</div></root>', []);
+
+// Test multiple elements with same ID (should return all)
+testIdFunction('id("duplicate")', '<root><div id="duplicate">First</div><div id="duplicate">Second</div></root>', ['duplicate', 'duplicate']);
+
+// Test IDs with special characters
+testIdFunction('id("test-1")', '<root><div id="test-1">Match</div></root>', ['test-1']);
+
+// Test empty ID string
+testIdFunction('id("")', '<root><div id="">Empty ID</div></root>', []);
+
+// Test whitespace in ID string
+testIdFunction('id(" test1 ")', '<root><div id="test1">Match</div></root>', ['test1']);
+</script>
+</body>