diff --git a/lib/attribute.js b/lib/attribute.js
index 49b5b1d..8979cf2 100644
--- a/lib/attribute.js
+++ b/lib/attribute.js
@@ -4,35 +4,25 @@
  * @typedef {import('./types.js').Node} Node
  */
 
-import {unreachable} from 'devlop'
-import {zwitch} from 'zwitch'
+import {ok as assert} from 'devlop'
 import {indexable} from './util.js'
 
-/** @type {(query: AstAttribute, node: Node) => boolean} */
-const handle = zwitch('operator', {
-  unknown: unknownOperator,
-  // @ts-expect-error: hush.
-  invalid: exists,
-  handlers: {
-    '=': exact,
-    '^=': begins,
-    '$=': ends,
-    '*=': containsString,
-    '~=': containsArray
-  }
-})
-
 /**
  * @param {AstRule} query
+ *   Query.
  * @param {Node} node
+ *   Node.
  * @returns {boolean}
+ *   Whether `node` matches `query`.
  */
-export function attribute(query, node) {
+export function attributes(query, node) {
   let index = -1
 
   if (query.attributes) {
     while (++index < query.attributes.length) {
-      if (!handle(query.attributes[index], node)) return false
+      if (!attribute(query.attributes[index], node)) {
+        return false
+      }
     }
   }
 
@@ -40,150 +30,67 @@ export function attribute(query, node) {
 }
 
 /**
- * Check whether an attribute exists.
- *
- * `[attr]`
- *
  * @param {AstAttribute} query
+ *   Query.
  * @param {Node} node
+ *   Node.
  * @returns {boolean}
+ *   Whether `node` matches `query`.
  */
-function exists(query, node) {
-  indexable(node)
-  return node[query.name] !== null && node[query.name] !== undefined
-}
 
-/**
- * Check whether an attribute has an exact value.
- *
- * `[attr=value]`
- *
- * @param {AstAttribute} query
- * @param {Node} node
- * @returns {boolean}
- */
-function exact(query, node) {
-  const queryValue = attributeValue(query)
-  indexable(node)
-  return exists(query, node) && String(node[query.name]) === queryValue
-}
-
-/**
- * Check whether an attribute, as a list, contains a value.
- *
- * When the attribute value is not a list, checks that the serialized value
- * is the queried one.
- *
- * `[attr~=value]`
- *
- * @param {AstAttribute} query
- * @param {Node} node
- * @returns {boolean}
- */
-function containsArray(query, node) {
+function attribute(query, node) {
   indexable(node)
   const value = node[query.name]
 
-  if (value === null || value === undefined) return false
-
-  const queryValue = attributeValue(query)
-
-  // If this is an array, and the query is contained in it, return `true`.
-  // Coverage comment in place because TS turns `Array.isArray(unknown)`
-  // into `Array<any>` instead of `Array<unknown>`.
-  // type-coverage:ignore-next-line
-  if (Array.isArray(value) && value.includes(queryValue)) {
-    return true
+  // Exists.
+  if (!query.value) {
+    return value !== null && value !== undefined
   }
 
-  // For all other values, return whether this is an exact match.
-  return String(value) === queryValue
-}
+  assert(query.value.type === 'String', 'expected plain string')
+  let key = query.value.value
+  let normal = value === null || value === undefined ? undefined : String(value)
 
-/**
- * Check whether an attribute has a substring as its start.
- *
- * `[attr^=value]`
- *
- * @param {AstAttribute} query
- * @param {Node} node
- * @returns {boolean}
- */
-function begins(query, node) {
-  indexable(node)
-  const value = node[query.name]
-  const queryValue = attributeValue(query)
+  // Case-sensitivity.
+  if (query.caseSensitivityModifier === 'i') {
+    key = key.toLowerCase()
 
-  return Boolean(
-    query.value &&
-      typeof value === 'string' &&
-      value.slice(0, queryValue.length) === queryValue
-  )
-}
-
-/**
- * Check whether an attribute has a substring as its end.
- *
- * `[attr$=value]`
- *
- * @param {AstAttribute} query
- * @param {Node} node
- * @returns {boolean}
- */
-function ends(query, node) {
-  indexable(node)
-  const value = node[query.name]
-  const queryValue = attributeValue(query)
-
-  return Boolean(
-    query.value &&
-      typeof value === 'string' &&
-      value.slice(-queryValue.length) === queryValue
-  )
-}
-
-/**
- * Check whether an attribute contains a substring.
- *
- * `[attr*=value]`
- *
- * @param {AstAttribute} query
- * @param {Node} node
- * @returns {boolean}
- */
-function containsString(query, node) {
-  indexable(node)
-  const value = node[query.name]
-  const queryValue = attributeValue(query)
-
-  return Boolean(
-    typeof value === 'string' && queryValue && value.includes(queryValue)
-  )
-}
-
-// Shouldn’t be called, parser throws an error instead.
-/**
- * @param {unknown} query
- * @returns {never}
- */
-/* c8 ignore next 4 */
-function unknownOperator(query) {
-  // @ts-expect-error: `operator` guaranteed.
-  throw new Error('Unknown operator `' + query.operator + '`')
-}
-
-/**
- * @param {AstAttribute} query
- * @returns {string}
- */
-function attributeValue(query) {
-  const queryValue = query.value
+    if (normal) {
+      normal = normal.toLowerCase()
+    }
+  }
 
-  /* c8 ignore next 4 -- never happens with our config */
-  if (!queryValue) unreachable('Attribute values should be defined')
-  if (queryValue.type === 'Substitution') {
-    unreachable('Substitutions are not enabled')
+  if (value !== undefined) {
+    switch (query.operator) {
+      // Exact.
+      case '=': {
+        return typeof normal === 'string' && key === normal
+      }
+
+      // Ends.
+      case '$=': {
+        return typeof value === 'string' && value.slice(-key.length) === key
+      }
+
+      // Contains.
+      case '*=': {
+        return typeof value === 'string' && value.includes(key)
+      }
+
+      // Begins.
+      case '^=': {
+        return typeof value === 'string' && key === value.slice(0, key.length)
+      }
+
+      // Space-separated list.
+      case '~=': {
+        // type-coverage:ignore-next-line -- some bug with TS.
+        return (Array.isArray(value) && value.includes(key)) || normal === key
+      }
+      // Other values are not yet supported by CSS.
+      // No default
+    }
   }
 
-  return queryValue.value
+  return false
 }
diff --git a/lib/test.js b/lib/test.js
index 065656d..53c6774 100644
--- a/lib/test.js
+++ b/lib/test.js
@@ -5,7 +5,7 @@
  * @typedef {import('./types.js').SelectState} SelectState
  */
 
-import {attribute} from './attribute.js'
+import {attributes} from './attribute.js'
 import {pseudo} from './pseudo.js'
 
 /**
@@ -28,7 +28,7 @@ export function test(query, node, index, parent, state) {
       (!query.tag ||
         query.tag.type === 'WildcardTag' ||
         query.tag.name === node.type) &&
-      (!query.attributes || attribute(query, node)) &&
+      (!query.attributes || attributes(query, node)) &&
       (!query.pseudoClasses || pseudo(query, node, index, parent, state))
   )
 }
diff --git a/test/matches.js b/test/matches.js
index 69538ea..4ab950d 100644
--- a/test/matches.js
+++ b/test/matches.js
@@ -639,6 +639,56 @@ test('select.matches()', async function (t) {
     }
   )
 
+  await t.test('attributes, case modifiers `[attr i]`', async function (t) {
+    await t.test(
+      'should throw when using a modifier in a wrong place',
+      async function () {
+        assert.throws(function () {
+          matches('[x y]', u('a'))
+        }, /Expected a valid attribute selector operator/)
+      }
+    )
+
+    await t.test(
+      'should throw when using an unknown modifier',
+      async function () {
+        assert.throws(function () {
+          matches('[x=y z]', u('a'))
+        }, /Unknown attribute case sensitivity modifier/)
+      }
+    )
+
+    await t.test(
+      'should match sensitively (default) with `s` (#1)',
+      async function () {
+        assert.ok(matches('[x=y s]', u('a', {x: 'y'})))
+      }
+    )
+
+    await t.test(
+      'should match sensitively (default) with `s` (#2)',
+      async function () {
+        assert.ok(!matches('[x=y s]', u('a', {x: 'Y'})))
+      }
+    )
+
+    await t.test('should match insensitively with `i` (#1)', async function () {
+      assert.ok(matches('[x=y i]', u('a', {x: 'y'})))
+    })
+
+    await t.test('should match insensitively with `i` (#2)', async function () {
+      assert.ok(matches('[x=y i]', u('a', {x: 'Y'})))
+    })
+
+    await t.test('should match insensitively with `i` (#3)', async function () {
+      assert.ok(matches('[x=Y i]', u('a', {x: 'y'})))
+    })
+
+    await t.test('should match insensitively with `i` (#4)', async function () {
+      assert.ok(matches('[x=Y i]', u('a', {x: 'Y'})))
+    })
+  })
+
   await t.test('pseudo-classes', async function (t) {
     await t.test(':is', async function (t) {
       await t.test(