There's no way to know what are the default values on a CSS Shader filter. By default I mean, values that if passed to the shader, will not disort the image in any way, so that it could be equivalent to the passthrough filter. There's a bug on the spec tracking the @rule, that might fix it by letting the page specify what are the default values. https://www.w3.org/Bugs/Public/show_bug.cgi?id=16861