diff --git a/content/canvas/src/WebGLContext.cpp b/content/canvas/src/WebGLContext.cpp
index 922e8168801..e23251b5e2c 100644
--- a/content/canvas/src/WebGLContext.cpp
+++ b/content/canvas/src/WebGLContext.cpp
@@ -156,6 +156,11 @@ WebGLContext::WebGLContext()
     mStencilWriteMaskFront = 0xffffffff;
     mStencilWriteMaskBack  = 0xffffffff;
 
+    mViewportX = 0;
+    mViewportY = 0;
+    mViewportWidth = 0;
+    mViewportHeight = 0;
+
     mScissorTestEnabled = 0;
     mDitherEnabled = 1;
     mRasterizerDiscardEnabled = 0; // OpenGL ES 3.0 spec p244
@@ -193,6 +198,7 @@ WebGLContext::WebGLContext()
 
     mAlreadyGeneratedWarnings = 0;
     mAlreadyWarnedAboutFakeVertexAttrib0 = false;
+    mAlreadyWarnedAboutViewportLargerThanDest = false;
     mMaxWarnings = Preferences::GetInt("webgl.max-warnings-per-context", 32);
     if (mMaxWarnings < -1)
     {
@@ -573,6 +579,8 @@ WebGLContext::SetDimensions(int32_t width, int32_t height)
 
     mWidth = width;
     mHeight = height;
+    mViewportWidth = width;
+    mViewportHeight = height;
     mResetLayer = true;
     mOptionsFrozen = true;
 
diff --git a/content/canvas/src/WebGLContext.h b/content/canvas/src/WebGLContext.h
index e7830532ca0..a6120b04402 100644
--- a/content/canvas/src/WebGLContext.h
+++ b/content/canvas/src/WebGLContext.h
@@ -1136,6 +1136,12 @@ protected:
     GLint mStencilClearValue;
     GLfloat mDepthClearValue;
 
+    GLint mViewportX;
+    GLint mViewportY;
+    GLsizei mViewportWidth;
+    GLsizei mViewportHeight;
+    bool mAlreadyWarnedAboutViewportLargerThanDest;
+
     nsCOMPtr<nsITimer> mContextRestorer;
     bool mAllowRestore;
     bool mContextLossTimerRunning;
diff --git a/content/canvas/src/WebGLContextGL.cpp b/content/canvas/src/WebGLContextGL.cpp
index c8eacad7de1..b3e3cac6eed 100644
--- a/content/canvas/src/WebGLContextGL.cpp
+++ b/content/canvas/src/WebGLContextGL.cpp
@@ -53,7 +53,7 @@ using namespace mozilla::gfx;
 static bool BaseTypeAndSizeFromUniformType(GLenum uType, GLenum *baseType, GLint *unitSize);
 static GLenum InternalFormatForFormatAndType(GLenum format, GLenum type, bool isGLES2);
 
-inline const WebGLRectangleObject *WebGLContext::FramebufferRectangleObject() const {
+const WebGLRectangleObject *WebGLContext::FramebufferRectangleObject() const {
     return mBoundFramebuffer ? mBoundFramebuffer->RectangleObject()
                              : static_cast<const WebGLRectangleObject*>(this);
 }
@@ -3067,6 +3067,11 @@ WebGLContext::Viewport(GLint x, GLint y, GLsizei width, GLsizei height)
 
     MakeContextCurrent();
     gl->fViewport(x, y, width, height);
+
+    mViewportX = x;
+    mViewportY = y;
+    mViewportWidth = width;
+    mViewportHeight = height;
 }
 
 void
diff --git a/content/canvas/src/WebGLContextVertices.cpp b/content/canvas/src/WebGLContextVertices.cpp
index 421037cea81..5b453ac7254 100644
--- a/content/canvas/src/WebGLContextVertices.cpp
+++ b/content/canvas/src/WebGLContextVertices.cpp
@@ -733,6 +733,20 @@ void WebGLContext::Draw_cleanup()
             }
         }
     }
+
+    // Let's check the viewport
+    const WebGLRectangleObject* rect = FramebufferRectangleObject();
+    if (rect) {
+        if (mViewportWidth > rect->Width() ||
+            mViewportHeight > rect->Height())
+        {
+            if (!mAlreadyWarnedAboutViewportLargerThanDest) {
+                GenerateWarning("Drawing to a destination rect smaller than the viewport rect. "
+                                "(This warning will only be given once)");
+                mAlreadyWarnedAboutViewportLargerThanDest = true;
+            }
+        }
+    }
 }
 
 /*