diff --git a/Cargo.lock b/Cargo.lock
index 3a20ddbcf88..53d76db0ce6 100644
--- a/Cargo.lock
+++ b/Cargo.lock
@@ -3915,6 +3915,7 @@ dependencies = [
  "range",
  "rayon",
  "script_layout_interface",
+ "selectors",
  "serde",
  "serde_json",
  "servo_arc",
diff --git a/components/layout_2020/Cargo.toml b/components/layout_2020/Cargo.toml
index 6e4b1ccb884..2abc3a751d2 100644
--- a/components/layout_2020/Cargo.toml
+++ b/components/layout_2020/Cargo.toml
@@ -40,6 +40,7 @@ pixels = { path = "../pixels" }
 range = { path = "../range" }
 rayon = { workspace = true }
 script_layout_interface = { workspace = true }
+selectors = { workspace = true }
 serde = { workspace = true }
 serde_json = { workspace = true }
 servo_arc = { workspace = true }
diff --git a/components/layout_2020/dom_traversal.rs b/components/layout_2020/dom_traversal.rs
index 3966edea383..f5f27fa7e08 100644
--- a/components/layout_2020/dom_traversal.rs
+++ b/components/layout_2020/dom_traversal.rs
@@ -8,6 +8,7 @@ use html5ever::{local_name, LocalName};
 use log::warn;
 use script_layout_interface::wrapper_traits::{ThreadSafeLayoutElement, ThreadSafeLayoutNode};
 use script_layout_interface::{LayoutElementType, LayoutNodeType};
+use selectors::Element as SelectorsElement;
 use servo_arc::Arc as ServoArc;
 use style::properties::ComputedValues;
 use style::selector_parser::PseudoElement;
@@ -413,8 +414,28 @@ where
                             .to_threadsafe()
                             .as_element()
                             .expect("Expected an element");
-                        let attr_val = element
-                            .get_attr(&attr.namespace_url, &LocalName::from(&*attr.attribute));
+
+                        // From
+                        // <https://html.spec.whatwg.org/multipage/#case-sensitivity-of-the-css-%27attr%28%29%27-function>
+                        //
+                        // > CSS Values and Units leaves the case-sensitivity of attribute names for
+                        // > the purpose of the `attr()` function to be defined by the host language.
+                        // > [[CSSVALUES]].
+                        // >
+                        // > When comparing the attribute name part of a CSS `attr()`function to the
+                        // > names of namespace-less attributes on HTML elements in HTML documents,
+                        // > the name part of the CSS `attr()` function must first be converted to
+                        // > ASCII lowercase. The same function when compared to other attributes must
+                        // > be compared according to its original case. In both cases, to match the
+                        // > values must be identical to each other (and therefore the comparison is
+                        // > case sensitive).
+                        let attr_name = match element.is_html_element_in_html_document() {
+                            true => &*attr.attribute.to_ascii_lowercase(),
+                            false => &*attr.attribute,
+                        };
+
+                        let attr_val =
+                            element.get_attr(&attr.namespace_url, &LocalName::from(attr_name));
                         vec.push(PseudoElementContentItem::Text(
                             attr_val.map_or("".to_string(), |s| s.to_string()),
                         ));
diff --git a/components/script/layout_dom/element.rs b/components/script/layout_dom/element.rs
index e4841b509a9..ec95cf0458b 100644
--- a/components/script/layout_dom/element.rs
+++ b/components/script/layout_dom/element.rs
@@ -847,8 +847,7 @@ impl<'dom> ::selectors::Element for ServoThreadSafeLayoutElement<'dom> {
     }
 
     fn is_html_element_in_html_document(&self) -> bool {
-        debug!("ServoThreadSafeLayoutElement::is_html_element_in_html_document called");
-        true
+        self.element.is_html_element_in_html_document()
     }
 
     #[inline]
diff --git a/tests/wpt/meta/css/CSS2/generated-content/content-attr-case-001.html.ini b/tests/wpt/meta/css/CSS2/generated-content/content-attr-case-001.html.ini
deleted file mode 100644
index 1cb6c0fdb8a..00000000000
--- a/tests/wpt/meta/css/CSS2/generated-content/content-attr-case-001.html.ini
+++ /dev/null
@@ -1,2 +0,0 @@
-[content-attr-case-001.html]
-  expected: FAIL
diff --git a/tests/wpt/meta/css/css-content/attr-case-sensitivity-001.html.ini b/tests/wpt/meta/css/css-content/attr-case-sensitivity-001.html.ini
deleted file mode 100644
index 18a4974b46c..00000000000
--- a/tests/wpt/meta/css/css-content/attr-case-sensitivity-001.html.ini
+++ /dev/null
@@ -1,2 +0,0 @@
-[attr-case-sensitivity-001.html]
-  expected: FAIL
diff --git a/tests/wpt/meta/css/css-content/attr-case-sensitivity-002.html.ini b/tests/wpt/meta/css/css-content/attr-case-sensitivity-002.html.ini
deleted file mode 100644
index c4c495049ee..00000000000
--- a/tests/wpt/meta/css/css-content/attr-case-sensitivity-002.html.ini
+++ /dev/null
@@ -1,2 +0,0 @@
-[attr-case-sensitivity-002.html]
-  expected: FAIL