diff --git a/dom/svg/SVGContentUtils.h b/dom/svg/SVGContentUtils.h
index 2a613d4899c2..26f96a7a5c52 100644
--- a/dom/svg/SVGContentUtils.h
+++ b/dom/svg/SVGContentUtils.h
@@ -39,6 +39,8 @@ class Matrix;
 } // namespace gfx
 } // namespace mozilla
 
+#define SVG_ZERO_LENGTH_PATH_FIX_FACTOR 512
+
 inline bool
 IsSVGWhitespace(char aChar)
 {
diff --git a/dom/svg/SVGLineElement.cpp b/dom/svg/SVGLineElement.cpp
index 7fc6a879ee47..8e481567f8f6 100644
--- a/dom/svg/SVGLineElement.cpp
+++ b/dom/svg/SVGLineElement.cpp
@@ -37,6 +37,23 @@ SVGLineElement::SVGLineElement(already_AddRefed<mozilla::dom::NodeInfo>& aNodeIn
 {
 }
 
+void
+SVGLineElement::MaybeAdjustForZeroLength(float aX1, float aY1,
+                                         float& aX2, float aY2)
+{
+  if (aX1 == aX2 && aY1 == aY2) {
+    SVGContentUtils::AutoStrokeOptions strokeOptions;
+    SVGContentUtils::GetStrokeOptions(&strokeOptions, this, nullptr, nullptr,
+                                      SVGContentUtils::eIgnoreStrokeDashing);
+
+    if (strokeOptions.mLineCap != CapStyle::BUTT) {
+      float tinyLength =
+        strokeOptions.mLineWidth / SVG_ZERO_LENGTH_PATH_FIX_FACTOR;
+      aX2 += tinyLength;
+    }
+  }
+}
+
 //----------------------------------------------------------------------
 // nsIDOMNode methods
 
@@ -112,6 +129,8 @@ SVGLineElement::GetAsSimplePath(SimplePath* aSimplePath)
 {
   float x1, y1, x2, y2;
   GetAnimatedLengthValues(&x1, &y1, &x2, &y2, nullptr);
+
+  MaybeAdjustForZeroLength(x1, y1, x2, y2);
   aSimplePath->SetLine(x1, y1, x2, y2);
 }
 
@@ -121,6 +140,7 @@ SVGLineElement::BuildPath(PathBuilder* aBuilder)
   float x1, y1, x2, y2;
   GetAnimatedLengthValues(&x1, &y1, &x2, &y2, nullptr);
 
+  MaybeAdjustForZeroLength(x1, y1, x2, y2);
   aBuilder->MoveTo(Point(x1, y1));
   aBuilder->LineTo(Point(x2, y2));
 
diff --git a/dom/svg/SVGLineElement.h b/dom/svg/SVGLineElement.h
index f80cd5cb57fe..93ae940130bf 100644
--- a/dom/svg/SVGLineElement.h
+++ b/dom/svg/SVGLineElement.h
@@ -25,6 +25,10 @@ protected:
   virtual JSObject* WrapNode(JSContext *cx, JS::Handle<JSObject*> aGivenProto) override;
   friend nsresult (::NS_NewSVGLineElement(nsIContent **aResult,
                                           already_AddRefed<mozilla::dom::NodeInfo>&& aNodeInfo));
+  // If the input line has length zero and linecaps aren't butt, adjust |aX2| by
+  // a tiny amount to a barely-nonzero-length line that all of our draw targets
+  // will render
+  void MaybeAdjustForZeroLength(float aX1, float aY1, float& aX2, float aY2);
 
 public:
   // nsIContent interface
diff --git a/dom/svg/SVGPathData.cpp b/dom/svg/SVGPathData.cpp
index 2294eb450968..2020798b4f5e 100644
--- a/dom/svg/SVGPathData.cpp
+++ b/dom/svg/SVGPathData.cpp
@@ -255,7 +255,7 @@ ApproximateZeroLengthSubpathSquareCaps(PathBuilder* aPB,
   // line is rather arbitrary, other than being chosen to meet the requirements
   // described in the comment above.
 
-  Float tinyLength = aStrokeWidth / 512;
+  Float tinyLength = aStrokeWidth / SVG_ZERO_LENGTH_PATH_FIX_FACTOR;
 
   aPB->LineTo(aPoint + Point(tinyLength, 0));
   aPB->MoveTo(aPoint);
diff --git a/layout/reftests/svg/stroke-linecap-round-w-zero-length-segs-01.svg b/layout/reftests/svg/stroke-linecap-round-w-zero-length-segs-01.svg
index 0f8e174d99b0..5a6c6ad2c21c 100644
--- a/layout/reftests/svg/stroke-linecap-round-w-zero-length-segs-01.svg
+++ b/layout/reftests/svg/stroke-linecap-round-w-zero-length-segs-01.svg
@@ -6,11 +6,12 @@
   
   <title>Test 'stroke-linecap: round' with zero length path segments</title>
 
-  <!-- From https://bugzilla.mozilla.org/show_bug.cgi?id=1075399 -->
+  <!-- From https://bugzilla.mozilla.org/show_bug.cgi?id=1075399 for paths and
+       https://bugzilla.mozilla.org/show_bug.cgi?id=1187770 for lines -->
 
   <style>
 
-path {
+path, line {
   stroke-width: 20px;
   stroke-linecap: round;
 }
@@ -20,7 +21,7 @@ circle {
 }
 
 /* expect lime circles to cover red circles */
-path.circles-expected {
+path.circles-expected, line.circles-expected {
   stroke: lime;
 }
 
@@ -158,4 +159,11 @@ path.coverer {
     <circle cy="80" r="8"/>
     <path class="circles-expected" d="M0,0v81" stroke-dasharray="0" />
   </g>
+
+  <!-- Column 6: test zero-length line -->
+  <g transform="translate(625, 25)">
+    <circle cy="0" r="8" />
+    <line class="circles-expected" x1="0" y1="0" x2="0" y2="0" />
+  </g>
+
 </svg>
diff --git a/layout/reftests/svg/stroke-linecap-round-w-zero-length-segs-02.svg b/layout/reftests/svg/stroke-linecap-round-w-zero-length-segs-02.svg
index ced221b42410..223a5e0df30e 100644
--- a/layout/reftests/svg/stroke-linecap-round-w-zero-length-segs-02.svg
+++ b/layout/reftests/svg/stroke-linecap-round-w-zero-length-segs-02.svg
@@ -6,13 +6,15 @@
   
   <title>Test invalidation for 'stroke-linecap: round' with zero length path segments</title>
 
-  <!-- From https://bugzilla.mozilla.org/show_bug.cgi?id=1075399 -->
+  <!-- From https://bugzilla.mozilla.org/show_bug.cgi?id=1075399 for path and
+       https://bugzilla.mozilla.org/show_bug.cgi?id=1187770 for line -->
 
   <script>
 
 function run()
 {
   document.getElementById('path').setAttribute('stroke-linecap', 'butt');
+  document.getElementById('line').setAttribute('stroke-linecap', 'butt');
   document.documentElement.removeAttribute('class');
 }
 
@@ -24,5 +26,7 @@ window.addEventListener("MozReftestInvalidate", run, false);
 
   <path id="path" stroke="red" stroke-width="200" stroke-linecap="round"
         d="M100,100 L100,100"/>
+  <line id="line" stroke="red" stroke-width="200" stroke-linecap="round"
+        x1="300" y1="300" x2="300" y2="300"/>
 
 </svg>
diff --git a/layout/reftests/svg/stroke-linecap-square-w-zero-length-segs-01.svg b/layout/reftests/svg/stroke-linecap-square-w-zero-length-segs-01.svg
index b7fdf4f429a2..df395b0e161b 100644
--- a/layout/reftests/svg/stroke-linecap-square-w-zero-length-segs-01.svg
+++ b/layout/reftests/svg/stroke-linecap-square-w-zero-length-segs-01.svg
@@ -6,11 +6,12 @@
   
   <title>Test 'stroke-linecap: square' with zero length path segments</title>
 
-  <!-- From https://bugzilla.mozilla.org/show_bug.cgi?id=589648 -->
+  <!-- From https://bugzilla.mozilla.org/show_bug.cgi?id=589648 for paths and
+       https://bugzilla.mozilla.org/show_bug.cgi?id=1187770 for lines -->
 
   <style>
 
-path {
+path, line {
   stroke-width: 20px;
   stroke-linecap: square;
 }
@@ -20,7 +21,7 @@ rect {
 }
 
 /* expect lime squares to cover red rects */
-path.squares-expected {
+path.squares-expected, line.squares-expected {
   stroke: lime;
 }
 
@@ -144,4 +145,10 @@ path.coverer {
     <path class="coverer" d="M20,20 L30,30 M70,70 L80,80"/>
   </g>
 
+  <!-- Column 5: test zero-length line -->
+  <g transform="translate(525, 25)">
+    <rect x="-9" y="-9" width="18" height="18"/>
+    <line class="squares-expected" x1="0" y1="0" x2="0" y2="0" />
+  </g>
+
 </svg>
diff --git a/layout/reftests/svg/stroke-linecap-square-w-zero-length-segs-02.svg b/layout/reftests/svg/stroke-linecap-square-w-zero-length-segs-02.svg
index 13c980abad74..506b2f12fdf6 100644
--- a/layout/reftests/svg/stroke-linecap-square-w-zero-length-segs-02.svg
+++ b/layout/reftests/svg/stroke-linecap-square-w-zero-length-segs-02.svg
@@ -6,13 +6,15 @@
   
   <title>Test 'stroke-linecap: square' with zero length path segments</title>
 
-  <!-- From https://bugzilla.mozilla.org/show_bug.cgi?id=589648 -->
+  <!-- From https://bugzilla.mozilla.org/show_bug.cgi?id=589648 for path and
+       https://bugzilla.mozilla.org/show_bug.cgi?id=1187770 for line -->
 
   <script>
 
 function run()
 {
   document.getElementById('path').setAttribute('stroke-linecap', 'butt');
+  document.getElementById('line').setAttribute('stroke-linecap', 'butt');
   document.documentElement.removeAttribute('class');
 }
 
@@ -25,4 +27,7 @@ window.addEventListener("MozReftestInvalidate", run, false);
   <path id="path" stroke="red" stroke-width="200" stroke-linecap="square"
         d="M100,100 L100,100"/>
 
+  <line id="line" stroke="red" stroke-width="200" stroke-linecap="square"
+        x1="300" y1="300" x2="300" y2="300"/>
+
 </svg>