convertToBlob() must be added to canvas color management proposal. The current IDL interface is as follows:

enum ImageColorSpace {
    "srgb", // default
    "rec2020",
    "display-p3",
};

enum ImagePixelFormat {
    "8-8-8-8", // default
    "uint16",
};

dictionary ImageEncodeOptions {
    DOMString type = "image/png";
    unrestricted double quality = 1.0;
    ImageColorSpace colorSpace = "srgb";
    ImagePixelFormat pixelFormat = "8-8-8-8";
};

Promise<Blob> convertToBlob(optional ImageEncodeOptions options);