--- a/dom/canvas/WebGLFormats.cpp
+++ b/dom/canvas/WebGLFormats.cpp
@@ -554,17 +554,17 @@ FormatUsageAuthority::CreateForWebGL1(gl
 
 #define FOO(x) ptr->AddRBFormat(LOCAL_GL_ ## x, ptr->GetUsage(EffectiveFormat::x))
 
     FOO(RGBA4            );
     FOO(RGB5_A1          );
     FOO(RGB565           );
     FOO(DEPTH_COMPONENT16);
     FOO(STENCIL_INDEX8   );
-    //FOO(DEPTH24_STENCIL8 );
+    //FOO(DEPTH24_STENCIL8 ); // WebGL 1 uses DEPTH_STENCIL instead of DEPTH24_STENCIL8.
 
 #undef FOO
 
     ptr->AddRBFormat(LOCAL_GL_DEPTH_STENCIL,
                      ptr->GetUsage(EffectiveFormat::DEPTH24_STENCIL8));
 
     ////////////////////////////////////////////////////////////////////////////
 

--- a/dom/canvas/WebGLTexelConversions.cpp
+++ b/dom/canvas/WebGLTexelConversions.cpp
@@ -372,17 +372,17 @@ ConvertImage(size_t width, size_t height
         //
         // The case where absolutely nothing needs to be done is supposed to have
         // been handled earlier (in TexImage2D_base, etc).
         //
         // So the case we're handling here is when even though no format conversion is
         // needed, we still might have to flip vertically and/or to adjust to a different
         // stride.
 
-        MOZ_ASSERT(!shouldYFlip || srcStride != dstStride,
+        MOZ_ASSERT(shouldYFlip || srcStride != dstStride,
                    "Performance trap -- should handle this case earlier to avoid memcpy");
 
         const auto bytesPerPixel = TexelBytesForFormat(srcFormat);
         const size_t bytesPerRow = bytesPerPixel * width;
 
         while (srcItr != srcEnd) {
             memcpy(dstItr, srcItr, bytesPerRow);
             srcItr += srcStride;

--- a/dom/canvas/WebGLTexture.cpp
+++ b/dom/canvas/WebGLTexture.cpp
@@ -800,17 +800,17 @@ WebGLTexture::TexParameter(TexTarget tex
     case LOCAL_GL_TEXTURE_MIN_LOD:
     case LOCAL_GL_TEXTURE_WRAP_R:
         if (mContext->IsWebGL2())
             isPNameValid = true;
         break;
 
     case LOCAL_GL_TEXTURE_MAX_ANISOTROPY_EXT:
         if (mContext->IsExtensionEnabled(WebGLExtensionID::EXT_texture_filter_anisotropic))
-            isPNameValid = false;
+            isPNameValid = true;
         break;
     }
 
     if (!isPNameValid) {
         mContext->ErrorInvalidEnumInfo("texParameter: pname", pname);
         return;
     }
 

--- a/dom/canvas/WebGLTextureUpload.cpp
+++ b/dom/canvas/WebGLTextureUpload.cpp
@@ -404,29 +404,34 @@ WebGLTexture::ValidateTexImageSpecificat
                                             GLsizei depth, GLint border,
                                             WebGLTexture::ImageInfo** const out_imageInfo)
 {
     if (mImmutable) {
         mContext->ErrorInvalidOperation("%s: Specified texture is immutable.", funcName);
         return false;
     }
 
+    // Do this early to validate `level`.
+    WebGLTexture::ImageInfo* imageInfo;
+    if (!ValidateTexImage(mContext, this, funcName, target, level, &imageInfo))
+        return false;
+
     // Check border
     if (border != 0) {
         mContext->ErrorInvalidValue("%s: `border` must be 0.", funcName);
         return false;
     }
 
-    if (level < 0 || width < 0 || height < 0 || depth < 0) {
+    if (width < 0 || height < 0 || depth < 0) {
         /* GL ES Version 2.0.25 - 3.7.1 Texture Image Specification
          *   "If wt and ht are the specified image width and height,
          *   and if either wt or ht are less than zero, then the error
          *   INVALID_VALUE is generated."
          */
-        mContext->ErrorInvalidValue("%s: `level`/`width`/`height`/`depth` must be >= 0.",
+        mContext->ErrorInvalidValue("%s: `width`/`height`/`depth` must be >= 0.",
                                     funcName);
         return false;
     }
 
     /* GLES 3.0.4, p133-134:
      * GL_MAX_TEXTURE_SIZE is *not* the max allowed texture size. Rather, it is the
      * max (width/height) size guaranteed not to generate an INVALID_VALUE for too-large
      * dimensions. Sizes larger than GL_MAX_TEXTURE_SIZE *may or may not* result in an
@@ -438,41 +443,40 @@ WebGLTexture::ValidateTexImageSpecificat
      *
      * Note that mImplMaxTextureSize must be >= than the advertized MAX_TEXTURE_SIZE.
      * For simplicity, we advertize MAX_TEXTURE_SIZE as mImplMaxTextureSize.
      */
 
     uint32_t maxWidthHeight = 0;
     uint32_t maxDepth = 0;
 
-    if (level <= 31) {
-        switch (target.get()) {
-        case LOCAL_GL_TEXTURE_2D:
-            maxWidthHeight = mContext->mImplMaxTextureSize >> level;
-            maxDepth = 1;
-            break;
+    MOZ_ASSERT(level <= 31);
+    switch (target.get()) {
+    case LOCAL_GL_TEXTURE_2D:
+        maxWidthHeight = mContext->mImplMaxTextureSize >> level;
+        maxDepth = 1;
+        break;
 
-        case LOCAL_GL_TEXTURE_3D:
-            maxWidthHeight = mContext->mImplMax3DTextureSize >> level;
-            maxDepth = maxWidthHeight;
-            break;
+    case LOCAL_GL_TEXTURE_3D:
+        maxWidthHeight = mContext->mImplMax3DTextureSize >> level;
+        maxDepth = maxWidthHeight;
+        break;
 
-        case LOCAL_GL_TEXTURE_2D_ARRAY:
-            maxWidthHeight = mContext->mImplMaxTextureSize >> level;
-            // "The maximum number of layers for two-dimensional array textures (depth)
-            //  must be at least MAX_ARRAY_TEXTURE_LAYERS for all levels."
-            maxDepth = mContext->mImplMaxArrayTextureLayers;
-            break;
+    case LOCAL_GL_TEXTURE_2D_ARRAY:
+        maxWidthHeight = mContext->mImplMaxTextureSize >> level;
+        // "The maximum number of layers for two-dimensional array textures (depth)
+        //  must be at least MAX_ARRAY_TEXTURE_LAYERS for all levels."
+        maxDepth = mContext->mImplMaxArrayTextureLayers;
+        break;
 
-        default: // cube maps
-            MOZ_ASSERT(IsCubeMap());
-            maxWidthHeight = mContext->mImplMaxCubeMapTextureSize >> level;
-            maxDepth = 1;
-            break;
-        }
+    default: // cube maps
+        MOZ_ASSERT(IsCubeMap());
+        maxWidthHeight = mContext->mImplMaxCubeMapTextureSize >> level;
+        maxDepth = 1;
+        break;
     }
 
     if (uint32_t(width) > maxWidthHeight ||
         uint32_t(height) > maxWidthHeight ||
         uint32_t(depth) > maxDepth)
     {
         mContext->ErrorInvalidValue("%s: Requested size at this level is unsupported.",
                                     funcName);
@@ -494,20 +498,16 @@ WebGLTexture::ValidateTexImageSpecificat
                 mContext->ErrorInvalidValue("%s: For level > 0, width and height must be"
                                             " powers of two.",
                                             funcName);
                 return false;
             }
         }
     }
 
-    WebGLTexture::ImageInfo* imageInfo;
-    if (!ValidateTexImage(mContext, this, funcName, target, level, &imageInfo))
-        return false;
-
     *out_imageInfo = imageInfo;
     return true;
 }
 
 // For *TexSubImage*
 bool
 WebGLTexture::ValidateTexImageSelection(const char* funcName, TexImageTarget target,
                                         GLint level, GLint xOffset, GLint yOffset,