https://github.com/KhronosGroup/WebGL/blob/65c5e7b16e98cbb998f742c45f9233f6f038c5c6/sdk/tests/js/tests/tex-input-validation.js#L443 Error message: "getError was expected value: NO_ERROR : colorBufferFormat: RGB5_A1 internalFormat: RGBA target: TEXTURE_2D" I think the root cause is ValidateCopyTexSubImage2D() return false when the RGB5_A1 colorBufferFormat doing CopyTexSubImage2D(). (https://dxr.mozilla.org/mozilla-central/rev/8103c612b79c2587ea4ca1b0a9f9f82db4b185b8/gfx/angle/src/libGLESv2/entry_points_gles_2_0.cpp#441) I have checked if we switch ValidateES3CopyTexImage2DParameters() with ValidateES2CopyTexImageParameters will solve our problem. In IsValidES3CopyTexImageCombination(), it will check if internalFormat is equal to textureFormat. if (!EqualOrFirstZero(textureFormatInfo.redBits, sourceEffectiveFormat->redBits) || !EqualOrFirstZero(textureFormatInfo.greenBits, sourceEffectiveFormat->greenBits) || !EqualOrFirstZero(textureFormatInfo.blueBits, sourceEffectiveFormat->blueBits) || !EqualOrFirstZero(textureFormatInfo.alphaBits, sourceEffectiveFormat->alphaBits)) { return false; } However, our internalFormat channels is (8, 8, 8, 8), sourceEffectiveFormat channels is (5, 5, 5, 1). In this case, it does not allow to do copy texture.