--- a/testing/web-platform/tests/url/urlencoded-parser.any.js
+++ b/testing/web-platform/tests/url/urlencoded-parser.any.js
@@ -23,17 +23,21 @@
   { "input": '&&&a=b&&&&c=d&', "output": [['a', 'b'], ['c', 'd']] },
   { "input": 'a=a&a=b&a=c', "output": [['a', 'a'], ['a', 'b'], ['a', 'c']] },
   { "input": 'a==a', "output": [['a', '=a']] },
   { "input": 'a=a+b+c+d', "output": [['a', 'a b c d']] },
   { "input": '%=a', "output": [['%', 'a']] },
   { "input": '%a=a', "output": [['%a', 'a']] },
   { "input": '%a_=a', "output": [['%a_', 'a']] },
   { "input": '%61=a', "output": [['a', 'a']] },
-  { "input": '%61+%4d%4D=', "output": [['a MM', '']] }
+  { "input": '%61+%4d%4D=', "output": [['a MM', '']] },
+  { "input": "id=0&value=%", "output": [['id', '0'], ['value', '%']] },
+  { "input": "b=%2sf%2a", "output": [['b', '%2sf*']]},
+  { "input": "b=%2%2af%2a", "output": [['b', '%2*f*']]},
+  { "input": "b=%%2a", "output": [['b', '%*']]}
 ].forEach((val) => {
   test(() => {
     let sp = new URLSearchParams(val.input),
         i = 0
     for (let item of sp) {
        assert_array_equals(item, val.output[i])
        i++
     }

--- a/testing/web-platform/tests/url/urlsearchparams-constructor.any.js
+++ b/testing/web-platform/tests/url/urlsearchparams-constructor.any.js
@@ -51,16 +51,38 @@ test(function() {
     params = new URLSearchParams('&a&&& &&&&&a+b=& c&m%c3%b8%c3%b8');
     assert_true(params != null, 'constructor returned non-null value.');
     assert_true(params.has('a'), 'Search params object has name "a"');
     assert_true(params.has('a b'), 'Search params object has name "a b"');
     assert_true(params.has(' '), 'Search params object has name " "');
     assert_false(params.has('c'), 'Search params object did not have the name "c"');
     assert_true(params.has(' c'), 'Search params object has name " c"');
     assert_true(params.has('møø'), 'Search params object has name "møø"');
+
+    params = new URLSearchParams('id=0&value=%');
+    assert_true(params != null, 'constructor returned non-null value.');
+    assert_true(params.has('id'), 'Search params object has name "id"');
+    assert_true(params.has('value'), 'Search params object has name "value"');
+    assert_equals(params.get('id'), '0');
+    assert_equals(params.get('value'), '%');
+
+    params = new URLSearchParams('b=%2sf%2a');
+    assert_true(params != null, 'constructor returned non-null value.');
+    assert_true(params.has('b'), 'Search params object has name "b"');
+    assert_equals(params.get('b'), '%2sf*');
+
+    params = new URLSearchParams('b=%2%2af%2a');
+    assert_true(params != null, 'constructor returned non-null value.');
+    assert_true(params.has('b'), 'Search params object has name "b"');
+    assert_equals(params.get('b'), '%2*f*');
+
+    params = new URLSearchParams('b=%%2a');
+    assert_true(params != null, 'constructor returned non-null value.');
+    assert_true(params.has('b'), 'Search params object has name "b"');
+    assert_equals(params.get('b'), '%*');
 }, 'URLSearchParams constructor, string.');
 
 test(function() {
     var seed = new URLSearchParams('a=b&c=d');
     var params = new URLSearchParams(seed);
     assert_true(params != null, 'constructor returned non-null value.');
     assert_equals(params.get('a'), 'b');
     assert_equals(params.get('c'), 'd');

--- a/testing/web-platform/tests/url/urlsearchparams-stringifier.any.js
+++ b/testing/web-platform/tests/url/urlsearchparams-stringifier.any.js
@@ -73,16 +73,19 @@ test(function() {
 
 test(function() {
     var params = new URLSearchParams();
     params.append('a', 'b%c');
     assert_equals(params + '', 'a=b%25c');
     params.delete('a');
     params.append('a%b', 'c');
     assert_equals(params + '', 'a%25b=c');
+
+    params = new URLSearchParams('id=0&value=%')
+    assert_equals(params + '', 'id=0&value=%25')
 }, 'Serialize %');
 
 test(function() {
     var params = new URLSearchParams();
     params.append('a', 'b\0c');
     assert_equals(params + '', 'a=b%00c');
     params.delete('a');
     params.append('a\0b', 'c');
@@ -102,16 +105,25 @@ test(function() {
     var params;
     params = new URLSearchParams('a=b&c=d&&e&&');
     assert_equals(params.toString(), 'a=b&c=d&e=');
     params = new URLSearchParams('a = b &a=b&c=d%20');
     assert_equals(params.toString(), 'a+=+b+&a=b&c=d+');
     // The lone '=' _does_ survive the roundtrip.
     params = new URLSearchParams('a=&a=b');
     assert_equals(params.toString(), 'a=&a=b');
+
+    params = new URLSearchParams('b=%2sf%2a');
+    assert_equals(params.toString(), 'b=%252sf*');
+
+    params = new URLSearchParams('b=%2%2af%2a');
+    assert_equals(params.toString(), 'b=%252*f*');
+
+    params = new URLSearchParams('b=%%2a');
+    assert_equals(params.toString(), 'b=%25*');
 }, 'URLSearchParams.toString');
 
 test(() => {
     const url = new URL('http://www.example.com/?a=b,c');
     const params = url.searchParams;
 
     assert_equals(url.toString(), 'http://www.example.com/?a=b,c');
     assert_equals(params.toString(), 'a=b%2Cc');