diff --git a/src/lexer.rs b/src/lexer.rs
index ae3622b..2367d16 100644
--- a/src/lexer.rs
+++ b/src/lexer.rs
@@ -4,7 +4,7 @@ use std::str::Chars;
 
 use crate::atrule::AtRuleKind;
 use crate::common::{Keyword, Op, Pos, Symbol};
-use crate::selector::{Attribute, AttributeKind};
+use crate::selector::{Attribute, AttributeKind, CaseKind};
 use crate::{Token, TokenKind, Whitespace};
 
 #[derive(Debug, Clone)]
@@ -243,7 +243,7 @@ impl<'a> Lexer<'a> {
                     kind: AttributeKind::Any,
                     attr,
                     value: String::new(),
-                    case_sensitive: true,
+                    case_sensitive: CaseKind::Sensitive,
                 })
             }
             'i' => {
@@ -253,7 +253,17 @@ impl<'a> Lexer<'a> {
                     kind: AttributeKind::Any,
                     attr,
                     value: String::new(),
-                    case_sensitive: false,
+                    case_sensitive: CaseKind::InsensitiveLowercase,
+                });
+            }
+            'I' => {
+                self.devour_whitespace();
+                assert!(self.buf.next() == Some(']'));
+                return TokenKind::Attribute(Attribute {
+                    kind: AttributeKind::Any,
+                    attr,
+                    value: String::new(),
+                    case_sensitive: CaseKind::InsensitiveCapital,
                 });
             }
             '=' => AttributeKind::Equals,
@@ -272,14 +282,19 @@ impl<'a> Lexer<'a> {
         self.devour_whitespace();
 
         let mut value = String::with_capacity(99);
-        let mut case_sensitive = true;
+        let mut case_sensitive = CaseKind::Sensitive;
 
         while let Some(c) = self.buf.peek() {
             if !c.is_alphabetic() && c != &'-' && c != &'_' && c != &'"' && c != &'\'' {
                 break;
             }
 
-            if c == &'i' {
+            if c == &'i' || c == &'I' {
+                if c == &'i' {
+                    case_sensitive = CaseKind::InsensitiveLowercase;
+                } else if c == &'I' {
+                    case_sensitive = CaseKind::InsensitiveCapital;
+                }
                 let tok = self
                     .buf
                     .next()
@@ -287,7 +302,14 @@ impl<'a> Lexer<'a> {
                 self.pos.next_char();
                 self.devour_whitespace();
                 match self.buf.next() {
-                    Some(']') => case_sensitive = false,
+                    Some(']') => {
+                        return TokenKind::Attribute(Attribute {
+                            kind,
+                            attr,
+                            value,
+                            case_sensitive,
+                        })
+                    }
                     Some(val) => {
                         self.pos.next_char();
                         value.push(tok);
@@ -304,6 +326,7 @@ impl<'a> Lexer<'a> {
                 .expect("this is impossible because we have already peeked");
             self.pos.next_char();
             value.push(tok);
+            self.devour_whitespace();
         }
 
         self.devour_whitespace();
diff --git a/src/selector.rs b/src/selector.rs
index 8993b34..778582b 100644
--- a/src/selector.rs
+++ b/src/selector.rs
@@ -329,38 +329,51 @@ impl Selector {
 }
 
 #[derive(Clone, Debug, Eq, PartialEq)]
-pub struct Attribute {
+pub(crate) struct Attribute {
     pub attr: String,
     pub value: String,
-    pub case_sensitive: bool,
+    pub case_sensitive: CaseKind,
     pub kind: AttributeKind,
 }
 
+#[derive(Clone, Debug, Eq, PartialEq)]
+pub(crate) enum CaseKind {
+    InsensitiveCapital,
+    InsensitiveLowercase,
+    Sensitive,
+}
+
+impl Display for CaseKind {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        match self {
+            Self::InsensitiveCapital => write!(f, " I"),
+            Self::InsensitiveLowercase => write!(f, " i"),
+            Self::Sensitive => write!(f, ""),
+        }
+    }
+}
+
 impl Display for Attribute {
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
-        if self.case_sensitive {
-            match self.kind {
-                AttributeKind::Any => write!(f, "[{}]", self.attr),
-                AttributeKind::Equals => write!(f, "[{}={}]", self.attr, self.value),
-                AttributeKind::InList => write!(f, "[{}~={}]", self.attr, self.value),
-                AttributeKind::BeginsWithHyphenOrExact => {
-                    write!(f, "[{}|={}]", self.attr, self.value)
-                }
-                AttributeKind::StartsWith => write!(f, "[{}^={}]", self.attr, self.value),
-                AttributeKind::EndsWith => write!(f, "[{}$={}]", self.attr, self.value),
-                AttributeKind::Contains => write!(f, "[{}*={}]", self.attr, self.value),
+        match self.kind {
+            AttributeKind::Any => write!(f, "[{}{}]", self.attr, self.case_sensitive),
+            AttributeKind::Equals => {
+                write!(f, "[{}={}{}]", self.attr, self.value, self.case_sensitive)
             }
-        } else {
-            match self.kind {
-                AttributeKind::Any => write!(f, "[{} i]", self.attr),
-                AttributeKind::Equals => write!(f, "[{}={} i]", self.attr, self.value),
-                AttributeKind::InList => write!(f, "[{}~={} i]", self.attr, self.value),
-                AttributeKind::BeginsWithHyphenOrExact => {
-                    write!(f, "[{}|={} i]", self.attr, self.value)
-                }
-                AttributeKind::StartsWith => write!(f, "[{}^={} i]", self.attr, self.value),
-                AttributeKind::EndsWith => write!(f, "[{}$={} i]", self.attr, self.value),
-                AttributeKind::Contains => write!(f, "[{}*={} i]", self.attr, self.value),
+            AttributeKind::InList => {
+                write!(f, "[{}~={}{}]", self.attr, self.value, self.case_sensitive)
+            }
+            AttributeKind::BeginsWithHyphenOrExact => {
+                write!(f, "[{}|={}{}]", self.attr, self.value, self.case_sensitive)
+            }
+            AttributeKind::StartsWith => {
+                write!(f, "[{}^={}{}]", self.attr, self.value, self.case_sensitive)
+            }
+            AttributeKind::EndsWith => {
+                write!(f, "[{}$={}{}]", self.attr, self.value, self.case_sensitive)
+            }
+            AttributeKind::Contains => {
+                write!(f, "[{}*={}{}]", self.attr, self.value, self.case_sensitive)
             }
         }
     }
diff --git a/tests/main.rs b/tests/main.rs
index 066a8f2..664e84c 100644
--- a/tests/main.rs
+++ b/tests/main.rs
@@ -127,6 +127,14 @@ mod test_selectors {
     );
 
     test!(selector_attribute_any, "[attr] {\n  color: red;\n}\n");
+    test!(
+        selector_attribute_any_lower_case_insensitive,
+        "[attr i] {\n  color: red;\n}\n"
+    );
+    test!(
+        selector_attribute_any_upper_case_insensitive,
+        "[attr I] {\n  color: red;\n}\n"
+    );
     test!(
         selector_attribute_equals,
         "[attr=val] {\n  color: red;\n}\n"