diff --git a/dom/html/HTMLInputElement.cpp b/dom/html/HTMLInputElement.cpp
index 9c09940757a9..0402d31aaa50 100644
--- a/dom/html/HTMLInputElement.cpp
+++ b/dom/html/HTMLInputElement.cpp
@@ -6422,22 +6422,22 @@ HTMLInputElement::SetRangeText(const nsAString& aReplacement, uint32_t aStart,
   SetSelectionRange(aSelectionStart, aSelectionEnd, direction, aRv);
 }
 
-Nullable<int32_t>
+Nullable<uint32_t>
 HTMLInputElement::GetSelectionStart(ErrorResult& aRv)
 {
   if (!SupportsTextSelection()) {
-    return Nullable<int32_t>();
+    return Nullable<uint32_t>();
   }
 
-  int32_t selStart = GetSelectionStartIgnoringType(aRv);
+  uint32_t selStart = GetSelectionStartIgnoringType(aRv);
   if (aRv.Failed()) {
-    return Nullable<int32_t>();
+    return Nullable<uint32_t>();
   }
 
-  return Nullable<int32_t>(selStart);
+  return Nullable<uint32_t>(selStart);
 }
 
-int32_t
+uint32_t
 HTMLInputElement::GetSelectionStartIgnoringType(ErrorResult& aRv)
 {
   int32_t selEnd, selStart;
@@ -6446,7 +6446,7 @@ HTMLInputElement::GetSelectionStartIgnoringType(ErrorResult& aRv)
 }
 
 void
-HTMLInputElement::SetSelectionStart(const Nullable<int32_t>& aSelectionStart,
+HTMLInputElement::SetSelectionStart(const Nullable<uint32_t>& aSelectionStart,
                                     ErrorResult& aRv)
 {
   if (!SupportsTextSelection()) {
@@ -6454,35 +6454,9 @@ HTMLInputElement::SetSelectionStart(const Nullable<int32_t>& aSelectionStart,
     return;
   }
 
-  int32_t selStart = 0;
-  if (!aSelectionStart.IsNull()) {
-    selStart = aSelectionStart.Value();
-  }
-
   nsTextEditorState* state = GetEditorState();
-  if (state && state->IsSelectionCached()) {
-    state->GetSelectionProperties().SetStart(selStart);
-    return;
-  }
-
-  nsAutoString direction;
-  GetSelectionDirection(direction, aRv);
-  if (aRv.Failed()) {
-    return;
-  }
-
-  int32_t start, end;
-  GetSelectionRange(&start, &end, aRv);
-  if (aRv.Failed()) {
-    return;
-  }
-
-  start = selStart;
-  if (end < start) {
-    end = start;
-  }
-
-  aRv = SetSelectionRange(start, end, direction);
+  MOZ_ASSERT(state, "SupportsTextSelection() returned true!");
+  state->SetSelectionStart(aSelectionStart, aRv);
 }
 
 Nullable<int32_t>
diff --git a/dom/html/HTMLInputElement.h b/dom/html/HTMLInputElement.h
index e9c46e46a404..2691e6065dd7 100644
--- a/dom/html/HTMLInputElement.h
+++ b/dom/html/HTMLInputElement.h
@@ -246,7 +246,7 @@ public:
 
   // Methods for nsFormFillController so it can do selection operations on input
   // types the HTML spec doesn't support them on, like "email".
-  int32_t GetSelectionStartIgnoringType(ErrorResult& aRv);
+  uint32_t GetSelectionStartIgnoringType(ErrorResult& aRv);
   int32_t GetSelectionEndIgnoringType(ErrorResult& aRv);
 
   void GetDisplayFileName(nsAString& aFileName) const;
@@ -717,8 +717,8 @@ public:
 
   // XPCOM Select() is OK
 
-  Nullable<int32_t> GetSelectionStart(ErrorResult& aRv);
-  void SetSelectionStart(const Nullable<int32_t>& aValue, ErrorResult& aRv);
+  Nullable<uint32_t> GetSelectionStart(ErrorResult& aRv);
+  void SetSelectionStart(const Nullable<uint32_t>& aValue, ErrorResult& aRv);
 
   Nullable<int32_t> GetSelectionEnd(ErrorResult& aRv);
   void SetSelectionEnd(const Nullable<int32_t>& aValue, ErrorResult& aRv);
diff --git a/dom/html/HTMLTextAreaElement.cpp b/dom/html/HTMLTextAreaElement.cpp
index 9ad6bf116a20..09e9af380b7e 100644
--- a/dom/html/HTMLTextAreaElement.cpp
+++ b/dom/html/HTMLTextAreaElement.cpp
@@ -710,34 +710,7 @@ void
 HTMLTextAreaElement::SetSelectionStart(const Nullable<uint32_t>& aSelectionStart,
                                        ErrorResult& aError)
 {
-  int32_t selStart = 0;
-  if (!aSelectionStart.IsNull()) {
-    selStart = aSelectionStart.Value();
-  }
-
-  if (mState.IsSelectionCached()) {
-    mState.GetSelectionProperties().SetStart(selStart);
-    return;
-  }
-
-  nsAutoString direction;
-  GetSelectionDirection(direction, aError);
-  if (aError.Failed()) {
-    return;
-  }
-  int32_t start, end;
-  GetSelectionRange(&start, &end, aError);
-  if (aError.Failed()) {
-    return;
-  }
-  start = selStart;
-  if (end < start) {
-    end = start;
-  }
-  nsresult rv = SetSelectionRange(start, end, direction);
-  if (NS_FAILED(rv)) {
-    aError.Throw(rv);
-  }
+  mState.SetSelectionStart(aSelectionStart, aError);
 }
 
 Nullable<uint32_t>
diff --git a/dom/html/nsTextEditorState.cpp b/dom/html/nsTextEditorState.cpp
index 1bdbe26f330a..8e51853fd4b6 100644
--- a/dom/html/nsTextEditorState.cpp
+++ b/dom/html/nsTextEditorState.cpp
@@ -1708,6 +1708,35 @@ nsTextEditorState::SetSelectionRange(int32_t aStart, int32_t aEnd,
   }
 }
 
+void
+nsTextEditorState::SetSelectionStart(const mozilla::dom::Nullable<uint32_t>& aStart,
+                                     ErrorResult& aRv)
+{
+  int32_t start = 0;
+  if (!aStart.IsNull()) {
+    // XXXbz This will do the wrong thing for input values that are out of the
+    // int32_t range...
+    start = aStart.Value();
+  }
+
+  int32_t ignored, end;
+  GetSelectionRange(&ignored, &end, aRv);
+  if (aRv.Failed()) {
+    return;
+  }
+
+  nsITextControlFrame::SelectionDirection dir = GetSelectionDirection(aRv);
+  if (aRv.Failed()) {
+    return;
+  }
+
+  if (end < start) {
+    end = start;
+  }
+
+  SetSelectionRange(start, end, dir, aRv);
+}
+
 HTMLInputElement*
 nsTextEditorState::GetParentNumberControl(nsFrame* aFrame) const
 {
diff --git a/dom/html/nsTextEditorState.h b/dom/html/nsTextEditorState.h
index 6d41b361fb2a..c26097490485 100644
--- a/dom/html/nsTextEditorState.h
+++ b/dom/html/nsTextEditorState.h
@@ -15,6 +15,7 @@
 #include "mozilla/Attributes.h"
 #include "mozilla/Maybe.h"
 #include "mozilla/WeakPtr.h"
+#include "mozilla/dom/Nullable.h"
 
 class nsTextInputListener;
 class nsTextControlFrame;
@@ -297,6 +298,12 @@ public:
                          nsITextControlFrame::SelectionDirection aDirection,
                          mozilla::ErrorResult& aRv);
 
+  // Set the selection start.  This basically implements the
+  // https://html.spec.whatwg.org/multipage/forms.html#dom-textarea/input-selectionstart
+  // setter.
+  void SetSelectionStart(const mozilla::dom::Nullable<uint32_t>& aStart,
+                         mozilla::ErrorResult& aRv);
+
   void UpdateEditableState(bool aNotify) {
     if (mRootNode) {
       mRootNode->UpdateEditableState(aNotify);
diff --git a/dom/webidl/HTMLInputElement.webidl b/dom/webidl/HTMLInputElement.webidl
index bdfc24ad6264..5f001e68739f 100644
--- a/dom/webidl/HTMLInputElement.webidl
+++ b/dom/webidl/HTMLInputElement.webidl
@@ -117,8 +117,7 @@ interface HTMLInputElement : HTMLElement {
   void select();
 
   [Throws]
-           // TODO: unsigned vs signed
-           attribute long? selectionStart;
+           attribute unsigned long? selectionStart;
   [Throws]
            attribute long? selectionEnd;
   [Throws]
diff --git a/testing/web-platform/meta/MANIFEST.json b/testing/web-platform/meta/MANIFEST.json
index b77d3c4c02a4..d4debe9920c9 100644
--- a/testing/web-platform/meta/MANIFEST.json
+++ b/testing/web-platform/meta/MANIFEST.json
@@ -93542,6 +93542,12 @@
      {}
     ]
    ],
+   "html/semantics/forms/textfieldselection/selection-start-end.html": [
+    [
+     "/html/semantics/forms/textfieldselection/selection-start-end.html",
+     {}
+    ]
+   ],
    "html/semantics/forms/textfieldselection/selection.html": [
     [
      "/html/semantics/forms/textfieldselection/selection.html",
@@ -176663,12 +176669,16 @@
    "39ecb031aca3655a06152f94a514981fe59ebbaf",
    "testharness"
   ],
+  "html/semantics/forms/textfieldselection/selection-start-end.html": [
+   "1f3184b72aba5631d6db4379dfa98035ee047283",
+   "testharness"
+  ],
   "html/semantics/forms/textfieldselection/selection.html": [
-   "d869799718137671a2eacc323aa26ea4364e845f",
+   "f7674721b84ec8fca0e5e40258447ce857b87784",
    "testharness"
   ],
   "html/semantics/forms/textfieldselection/textfieldselection-setRangeText.html": [
-   "0caf2b08ccc5a35578291af8f5adaf7de9537d66",
+   "3bbd350321f5ec9e0a8f3d47da4e11aaa3ad4d68",
    "testharness"
   ],
   "html/semantics/forms/textfieldselection/textfieldselection-setSelectionRange.html": [
diff --git a/testing/web-platform/tests/html/semantics/forms/textfieldselection/selection-start-end.html b/testing/web-platform/tests/html/semantics/forms/textfieldselection/selection-start-end.html
new file mode 100644
index 000000000000..49b598ec8dfc
--- /dev/null
+++ b/testing/web-platform/tests/html/semantics/forms/textfieldselection/selection-start-end.html
@@ -0,0 +1,68 @@
+<!doctype html>
+<meta charset=utf-8>
+<title></title>
+<script src=/resources/testharness.js></script>
+<script src=/resources/testharnessreport.js></script>
+<div id=log></div>
+<script>
+  function createInputElement(value, append) {
+    var el = document.createElement("input");
+    el.type = "text";
+    el.value = value;
+    el.id = "input" + (append ? "-appended" : "-not-appended");
+    if (append) {
+      document.body.appendChild(el);
+    }
+    return el;
+  };
+
+  function createTextareaElement(value, append) {
+    var el = document.createElement("textarea");
+    el.value = value;
+
+    el.id = "textarea" + (append ? "-appended" : "-not-appended");
+    if (append) {
+      document.body.appendChild(el);
+    }
+    return el;
+  };
+
+  function createTestElements(value) {
+    return [ createInputElement(value, true),
+             createInputElement(value, false),
+             createTextareaElement(value, true),
+             createTextareaElement(value, false) ];
+  }
+
+  const testValue = "abcdefghij";
+
+  test(function() {
+    assert_equals(testValue.length, 10);
+  }, "Sanity check for testValue length; if this fails, variou absolute offsets in the test below need to be adjusted to be less than testValue.length");
+
+  test(function() {
+    for (let el of createTestElements(testValue)) {
+      assert_equals(el.selectionStart, testValue.length,
+                    `Initial .value set on ${el.id} should set selectionStart to end of value`);
+      var t = async_test(`onselect should fire when selectionStart is changed on ${el.id}`);
+      el.onselect = t.step_func_done(function(e) {
+        assert_equals(e.type, "select");
+        el.remove();
+      });
+      el.selectionStart = 2;
+    }
+  }, "onselect should fire when selectionStart is changed");
+
+  test(function() {
+    for (let el of createTestElements(testValue)) {
+      assert_equals(el.selectionStart, testValue.length,
+                    `Initial .value set on ${el.id} should set selectionStart to end of value`);
+      el.selectionStart = 0;
+      el.selectionEnd = 5;
+      el.selectionStart = 8;
+      assert_equals(el.selectionStart, 8, `selectionStart on ${el.id}`);
+      assert_equals(el.selectionEnd, 8, `selectionEnd on ${el.id}`);
+      el.remove();
+    }
+  }, "Setting selectionStart to a value larger than selectionEnd should increase selectionEnd");
+</script>
diff --git a/testing/web-platform/tests/html/semantics/forms/textfieldselection/textfieldselection-setRangeText.html b/testing/web-platform/tests/html/semantics/forms/textfieldselection/textfieldselection-setRangeText.html
index cb0afb17d734..4b586cc10785 100644
--- a/testing/web-platform/tests/html/semantics/forms/textfieldselection/textfieldselection-setRangeText.html
+++ b/testing/web-platform/tests/html/semantics/forms/textfieldselection/textfieldselection-setRangeText.html
@@ -106,15 +106,23 @@
     }, element.id + " setRangeText without argument throws a type error");
 
     async_test(function() {
-      var q = false;
-      element.onselect = this.step_func_done(function(e) {
-        assert_true(q, "event should be queued");
-        assert_true(e.isTrusted, "event is trusted");
-        assert_true(e.bubbles, "event bubbles");
-        assert_false(e.cancelable, "event is not cancelable");
-      });
-      element.setRangeText("foobar2", 0, 6);
-      q = true;
+      // At this point there are already "select" events queued up on
+      // "element".  Give them time to fire; otherwise we can get spurious
+      // passes.
+      //
+      // This is unfortunately racy in that we might _still_ get spurious
+      // passes.  I'm not sure how best to handle that.
+      setTimeout(this.step_func(function() {
+        var q = false;
+        element.onselect = this.step_func_done(function(e) {
+          assert_true(q, "event should be queued");
+          assert_true(e.isTrusted, "event is trusted");
+          assert_true(e.bubbles, "event bubbles");
+          assert_false(e.cancelable, "event is not cancelable");
+        });
+        element.setRangeText("foobar2", 0, 6);
+        q = true;
+      }), 10);
     }, element.id + " setRangeText fires a select event");
   })
 </script>