diff --git a/components/script/dom/bindings/str.rs b/components/script/dom/bindings/str.rs
index 67ba6f34b3d..986554206b6 100644
--- a/components/script/dom/bindings/str.rs
+++ b/components/script/dom/bindings/str.rs
@@ -474,7 +474,15 @@ impl DOMString {
                 "{:04}-{:02}-{:02}T{:02}:{:02}",
                 year, month, day, hour, minute
             );
+        } else if second < 10.0 {
+            // we need exactly one leading zero on the seconds,
+            // whatever their total string length might be
+            self.0 = format!(
+                "{:04}-{:02}-{:02}T{:02}:{:02}:0{}",
+                year, month, day, hour, minute, second
+            );
         } else {
+            // we need no leading zeroes on the seconds
             self.0 = format!(
                 "{:04}-{:02}-{:02}T{:02}:{:02}:{}",
                 year, month, day, hour, minute, second
diff --git a/tests/wpt/metadata/MANIFEST.json b/tests/wpt/metadata/MANIFEST.json
index b63cdb025a4..8f37ab58752 100644
--- a/tests/wpt/metadata/MANIFEST.json
+++ b/tests/wpt/metadata/MANIFEST.json
@@ -369990,6 +369990,12 @@
      {}
     ]
    ],
+   "html/semantics/forms/the-input-element/input-seconds-leading-zeroes.html": [
+    [
+     "html/semantics/forms/the-input-element/input-seconds-leading-zeroes.html",
+     {}
+    ]
+   ],
    "html/semantics/forms/the-input-element/input-setcustomvalidity.html": [
     [
      "html/semantics/forms/the-input-element/input-setcustomvalidity.html",
@@ -693507,6 +693513,10 @@
    "006a8fbd8f1b84d18e9ec29285f751f037118389",
    "testharness"
   ],
+  "html/semantics/forms/the-input-element/input-seconds-leading-zeroes.html": [
+   "d94bee10296da58ba700b4e7a910f9e0821c82ef",
+   "testharness"
+  ],
   "html/semantics/forms/the-input-element/input-setcustomvalidity.html": [
    "accb24d8f9564f97e12bf4784162ff3cd00f35af",
    "testharness"
diff --git a/tests/wpt/web-platform-tests/html/semantics/forms/the-input-element/input-seconds-leading-zeroes.html b/tests/wpt/web-platform-tests/html/semantics/forms/the-input-element/input-seconds-leading-zeroes.html
new file mode 100644
index 00000000000..d94bee10296
--- /dev/null
+++ b/tests/wpt/web-platform-tests/html/semantics/forms/the-input-element/input-seconds-leading-zeroes.html
@@ -0,0 +1,52 @@
+<!DOCTYPE HTML>
+<meta charset="utf-8">
+<html>
+ <head>
+  <title>HTMLInputElement leading zeroes in seconds/millis</title>
+  <script src="/resources/testharness.js"></script>
+  <script src="/resources/testharnessreport.js"></script>
+ </head>
+ <body>
+  <h3>input times and datetimes with leading zeroes in seconds/millis</h3>
+  <!-- This test ensures that seconds and milliseconds are being
+       output with the appropriate field widths when sanitizing
+       datetime-locals and times, e.g. that we don't see "12:30:1".
+       The spec is not specific about how much precision to use
+       in a sanitized time string, but an invalid string would
+       fail at .valueAsNumber -->
+  <hr>
+  <div id="log"></div>
+
+  <input id="inp">
+  <script>
+    var inp=document.getElementById("inp");
+    var cases = [
+      ["datetime-local", "2000-01-01T12:30:01",     946729801000],
+      ["datetime-local", "2000-01-01T12:30:00.5",   946729800500],
+      ["datetime-local", "2000-01-01T12:30:00.04",  946729800040],
+      ["datetime-local", "2000-01-01T12:30:00.003", 946729800003],
+
+      ["time", "12:30:01",     45001000],
+      ["time", "12:30:00.5",   45000500],
+      ["time", "12:30:00.04",  45000040],
+      ["time", "12:30:00.003", 45000003],
+    ];
+
+    for (var i in cases) {
+      var c = cases[i];
+      test(function() {
+        inp.setAttribute("type", c[0]);
+        inp.value = c[1];
+        assert_equals(inp.valueAsNumber, c[2]);
+      },"Expected valueAsNumber=" +c[2] + " from " + c[1]);
+      if (c[0] == "datetime-local") {
+        test(function() {
+          inp.setAttribute("type", c[0]);
+          inp.value = c[1];
+          assert_in_array(inp.value, [c[1], c[1].replace("T", " ")]);
+        },"Expected digits unchanged in round-trip of " + c[1])
+      }
+    }
+  </script>
+ </body>
+</html>
