diff --git a/Tests/LibWeb/Text/expected/URL/invalid-urls.txt b/Tests/LibWeb/Text/expected/URL/invalid-urls.txt
new file mode 100644
index 00000000000..7ffbe3d1b7b
--- /dev/null
+++ b/Tests/LibWeb/Text/expected/URL/invalid-urls.txt
@@ -0,0 +1,2 @@
+new URL('file://xn--/p', undefined)
+error creating URL: 'TypeError: Invalid URL'
diff --git a/Tests/LibWeb/Text/input/URL/invalid-urls.html b/Tests/LibWeb/Text/input/URL/invalid-urls.html
new file mode 100644
index 00000000000..b8f73c9fece
--- /dev/null
+++ b/Tests/LibWeb/Text/input/URL/invalid-urls.html
@@ -0,0 +1,21 @@
+<script src="../include.js"></script>
+<script>
+    test(() => {
+        const urls = [
+            { input: 'file://xn--/p' },
+        ];
+
+        for (url of urls) {
+            if (url.base === undefined)
+                println(`new URL('${url.input}', ${url.base})`);
+            else
+                println(`new URL('${url.input}', '${url.base}')`);
+
+            try {
+                new URL(url.input, url.base);
+            } catch (e) {
+                println(`error creating URL: '${e}'`);
+            }
+        }
+    });
+</script>
diff --git a/Userland/Libraries/LibURL/Parser.cpp b/Userland/Libraries/LibURL/Parser.cpp
index 36a67998c39..6a9aa801b86 100644
--- a/Userland/Libraries/LibURL/Parser.cpp
+++ b/Userland/Libraries/LibURL/Parser.cpp
@@ -596,14 +596,26 @@ static ErrorOr<String> domain_to_ascii(StringView domain, bool be_strict)
     // 1. Let result be the result of running Unicode ToASCII with domain_name set to domain, UseSTD3ASCIIRules set to beStrict, CheckHyphens set to false, CheckBidi set to true, CheckJoiners set to true, Transitional_Processing set to false, and VerifyDnsLength set to beStrict. [UTS46]
     // 2. If result is a failure value, domain-to-ASCII validation error, return failure.
 
-    // OPTIMIZATION: Fast path for all-ASCII domain strings.
-    if (all_of(domain, is_ascii)) {
+    // OPTIMIZATION: If beStrict is false, domain is an ASCII string, and strictly splitting domain on U+002E (.)
+    //               does not produce any item that starts with an ASCII case-insensitive match for "xn--", this
+    //               step is equivalent to ASCII lowercasing domain.
+    if (!be_strict && all_of(domain, is_ascii)) {
         // 3. If result is the empty string, domain-to-ASCII validation error, return failure.
         if (domain.is_empty())
             return Error::from_string_literal("Empty domain");
 
-        auto lowercase_domain = domain.to_lowercase_string();
-        return String::from_utf8_without_validation(lowercase_domain.bytes());
+        bool slow_path = false;
+        for (auto part : domain.split_view('.')) {
+            if (part.starts_with("xn--"sv, CaseSensitivity::CaseInsensitive)) {
+                slow_path = true;
+                break;
+            }
+        }
+
+        if (!slow_path) {
+            auto lowercase_domain = domain.to_lowercase_string();
+            return String::from_utf8_without_validation(lowercase_domain.bytes());
+        }
     }
 
     Unicode::IDNA::ToAsciiOptions const options {