diff --git a/dom/canvas/WebGLContextExtensions.cpp b/dom/canvas/WebGLContextExtensions.cpp
index fe9b6fcccd63..749846d97175 100644
--- a/dom/canvas/WebGLContextExtensions.cpp
+++ b/dom/canvas/WebGLContextExtensions.cpp
@@ -184,7 +184,7 @@ WebGLContext::IsExtensionSupported(WebGLExtensionID ext) const
         case WebGLExtensionID::EXT_frag_depth:
             return WebGLExtensionFragDepth::IsSupported(this);
         case WebGLExtensionID::EXT_shader_texture_lod:
-            return gl->IsSupported(gl::GLFeature::shader_texture_lod);
+            return WebGLExtensionShaderTextureLod::IsSupported(this);
         case WebGLExtensionID::EXT_sRGB:
             return WebGLExtensionSRGB::IsSupported(this);
 
diff --git a/dom/canvas/WebGLExtensionShaderTextureLod.cpp b/dom/canvas/WebGLExtensionShaderTextureLod.cpp
index 55e023a7d588..40a70b4118ed 100644
--- a/dom/canvas/WebGLExtensionShaderTextureLod.cpp
+++ b/dom/canvas/WebGLExtensionShaderTextureLod.cpp
@@ -14,12 +14,25 @@ namespace mozilla {
 WebGLExtensionShaderTextureLod::WebGLExtensionShaderTextureLod(WebGLContext* webgl)
     : WebGLExtensionBase(webgl)
 {
+    MOZ_ASSERT(IsSupported(webgl), "Don't construct extension if unsupported.");
 }
 
 WebGLExtensionShaderTextureLod::~WebGLExtensionShaderTextureLod()
 {
 }
 
+bool
+WebGLExtensionShaderTextureLod::IsSupported(const WebGLContext* webgl)
+{
+    gl::GLContext* gl = webgl->GL();
+    if (gl->IsGLES() && gl->Version() >= 300) {
+        // ANGLE's shader translator doesn't yet translate
+        // WebGL1+EXT_shader_texture_lod to ES3. (Bug 1491221)
+        return false;
+    }
+    return gl->IsSupported(gl::GLFeature::shader_texture_lod);
+}
+
 IMPL_WEBGL_EXTENSION_GOOP(WebGLExtensionShaderTextureLod, EXT_shader_texture_lod)
 
 } // namespace mozilla
diff --git a/dom/canvas/WebGLExtensions.h b/dom/canvas/WebGLExtensions.h
index 5bce344c1242..cd2da32c75fb 100644
--- a/dom/canvas/WebGLExtensions.h
+++ b/dom/canvas/WebGLExtensions.h
@@ -251,6 +251,8 @@ public:
     explicit WebGLExtensionShaderTextureLod(WebGLContext*);
     virtual ~WebGLExtensionShaderTextureLod();
 
+    static bool IsSupported(const WebGLContext* context);
+
     DECL_WEBGL_EXTENSION_GOOP
 };