Per May 2016 TC39 consensus, String#{padStart,padEnd} is now stage 4!

spec.html

@@ -27130,6 +27130,46 @@ <h1>String.prototype [ @@iterator ] ( )</h1>
       </emu-clause>
     </emu-clause>
 
+    <emu-clause id="sec-string.prototype.padstart">
+      <h1>String.prototype.padStart( maxLength [ , fillString ] )</h1>
+      <p>When the `padStart` method is called, the following steps are taken:</p>
+      <emu-alg>
+        1. Let _O_ be ? RequireObjectCoercible(*this* value).
+        1. Let _S_ be ? ToString(_O_).
+        1. Let _intMaxLength_ be ? ToLength(_maxLength_).
+        1. Let _stringLength_ be the number of elements in _S_.
+        1. If _intMaxLength_ is not greater than _stringLength_, return _S_.
+        1. If _fillString_ is *undefined*, let _filler_ be a String consisting solely of the code unit 0x0020 (SPACE).
+        1. Else, let _filler_ be ? ToString(_fillString_).
+        1. If _filler_ is the empty String, return _S_.
+        1. Let _fillLen_ be _intMaxLength_ - _stringLength_.
+        1. Let _truncatedStringFiller_ be a new String value consisting of repeated concatenations of _filler_ truncated to length _fillLen_.
+        1. Return a new String value computed by the concatenation of _truncatedStringFiller_ and _S_.
+      </emu-alg>
+      <emu-note>The first argument _maxLength_ will be clamped such that it can be no smaller than the length of the *this* value.</emu-note>
+      <emu-note>The optional second argument _fillString_ defaults to *" "* (a String consisting of 0x0020 SPACE).</emu-note>
+    </emu-clause>
+
+    <emu-clause id="sec-string.prototype.padend">
+      <h1>String.prototype.padEnd( maxLength [ , fillString ] )</h1>
+      <p>When the `padEnd` method is called, the following steps are taken:</p>
+      <emu-alg>
+        1. Let _O_ be ? RequireObjectCoercible(*this* value).
+        1. Let _S_ be ? ToString(_O_).
+        1. Let _intMaxLength_ be ? ToLength(_maxLength_).
+        1. Let _stringLength_ be the number of elements in _S_.
+        1. If _intMaxLength_ is not greater than _stringLength_, return _S_.
+        1. If _fillString_ is *undefined*, let _filler_ be a String consisting solely of the code unit 0x0020 (SPACE).
+        1. Else, let _filler_ be ? ToString(_fillString_).
+        1. If _filler_ is the empty String, return _S_.
+        1. Let _fillLen_ be _intMaxLength_ - _stringLength_.
+        1. Let _truncatedStringFiller_ be a new String value consisting of repeated concatenations of _filler_ truncated to length _fillLen_.
+        1. Return a new String value computed by the concatenation of _S_ and _truncatedStringFiller_.
+      </emu-alg>
+      <emu-note>The first argument _maxLength_ will be clamped such that it can be no smaller than the length of the *this* value.</emu-note>
+      <emu-note>The optional second argument _fillString_ defaults to *" "* (a String consisting of 0x0020 SPACE).</emu-note>
+    </emu-clause>
+
     <!-- es6num="21.1.4" -->
     <emu-clause id="sec-properties-of-string-instances">
       <h1>Properties of String Instances</h1>