diff --git a/.gitignore b/.gitignore
index f19ff24..ce84e07 100644
--- a/.gitignore
+++ b/.gitignore
@@ -7,3 +7,4 @@ node_modules/
 .DS_Store
 react-markdown.min.js
 yarn.lock
+!/test-types.d.ts
diff --git a/lib/index.js b/lib/index.js
index d3d201f..529639c 100644
--- a/lib/index.js
+++ b/lib/index.js
@@ -318,11 +318,11 @@ export function defaultUrlTransform(value) {
 
   if (
     // If there is no protocol, it’s relative.
-    colon < 0 ||
+    colon === -1 ||
     // If the first colon is after a `?`, `#`, or `/`, it’s not a protocol.
-    (slash > -1 && colon > slash) ||
-    (questionMark > -1 && colon > questionMark) ||
-    (numberSign > -1 && colon > numberSign) ||
+    (slash !== -1 && colon > slash) ||
+    (questionMark !== -1 && colon > questionMark) ||
+    (numberSign !== -1 && colon > numberSign) ||
     // It is a protocol, it should be allowed.
     safeProtocol.test(value.slice(0, colon))
   ) {
diff --git a/package.json b/package.json
index c051f5e..b107383 100644
--- a/package.json
+++ b/package.json
@@ -94,14 +94,14 @@
   },
   "devDependencies": {
     "@types/node": "^22.0.0",
-    "@types/react": "^18.0.0",
-    "@types/react-dom": "^18.0.0",
+    "@types/react": "^19.0.0",
+    "@types/react-dom": "^19.0.0",
     "c8": "^10.0.0",
     "esbuild": "^0.24.0",
     "eslint-plugin-react": "^7.0.0",
     "prettier": "^3.0.0",
-    "react": "^18.0.0",
-    "react-dom": "^18.0.0",
+    "react": "^19.0.0",
+    "react-dom": "^19.0.0",
     "rehype-raw": "^7.0.0",
     "remark-cli": "^12.0.0",
     "remark-gfm": "^4.0.0",
@@ -109,7 +109,7 @@
     "remark-toc": "^9.0.0",
     "type-coverage": "^2.0.0",
     "typescript": "^5.0.0",
-    "xo": "^0.59.0"
+    "xo": "^0.60.0"
   },
   "scripts": {
     "build": "tsc --build --clean && tsc --build && type-coverage",
diff --git a/test-types.d.ts b/test-types.d.ts
new file mode 100644
index 0000000..4dd67e3
--- /dev/null
+++ b/test-types.d.ts
@@ -0,0 +1,9 @@
+import type {JSX as Jsx} from 'react/jsx-runtime'
+
+declare global {
+  namespace JSX {
+    type ElementClass = Jsx.ElementClass
+    type Element = Jsx.Element
+    type IntrinsicElements = Jsx.IntrinsicElements
+  }
+}
diff --git a/test-types.js b/test-types.js
new file mode 100644
index 0000000..f3ea4d4
--- /dev/null
+++ b/test-types.js
@@ -0,0 +1,2 @@
+// See `test-types.d.ts`.
+export {}
diff --git a/test.jsx b/test.jsx
index 8223918..f127de1 100644
--- a/test.jsx
+++ b/test.jsx
@@ -167,21 +167,24 @@ test('react-markdown', async function (t) {
   await t.test('should support an image', function () {
     assert.equal(
       renderToStaticMarkup(<Markdown children="![a](b)" />),
-      '<p><img src="b" alt="a"/></p>'
+      // Note: React weirdly adds `rel="preload"`.
+      '<link rel="preload" as="image" href="b"/><p><img src="b" alt="a"/></p>'
     )
   })
 
   await t.test('should support an image w/ a title', function () {
     assert.equal(
       renderToStaticMarkup(<Markdown children="![a](b (c))" />),
-      '<p><img src="b" alt="a" title="c"/></p>'
+      // Note: React weirdly adds `rel="preload"`.
+      '<link rel="preload" as="image" href="b"/><p><img src="b" alt="a" title="c"/></p>'
     )
   })
 
   await t.test('should support an image reference / definition', function () {
     assert.equal(
       renderToStaticMarkup(<Markdown children={'![a]\n\n[a]: b'} />),
-      '<p><img src="b" alt="a"/></p>'
+      // Note: React weirdly adds `rel="preload"`.
+      '<link rel="preload" as="image" href="b"/><p><img src="b" alt="a"/></p>'
     )
   })
 
@@ -410,7 +413,7 @@ test('react-markdown', async function (t) {
           }}
         />
       ),
-      '<p><img src="" alt="a" title="c"/></p>'
+      '<p><img alt="a" title="c"/></p>'
     )
   })
 
@@ -564,7 +567,7 @@ test('react-markdown', async function (t) {
 
     console.error = warn
 
-    assert.match(String(message), /Warning: React.jsx: type is invalid/)
+    assert.match(String(message), /type is invalid/)
 
     /**
      * @param {unknown} d
diff --git a/tsconfig.json b/tsconfig.json
index d13dc27..5317638 100644
--- a/tsconfig.json
+++ b/tsconfig.json
@@ -13,5 +13,5 @@
     "target": "es2022"
   },
   "exclude": ["coverage/", "node_modules/"],
-  "include": ["**/*.js", "**/*.jsx", "lib/complex-types.d.ts"]
+  "include": ["**/*.js", "**/*.jsx", "test-types.d.ts"]
 }