Nice. I do that as well, but with WebP only (WebP’s header is smaller than JPEG, I remember that Facebook article about their app reconstructing the JPEG header…).

So WebP-supporting browsers get the tiny base64 preview and the full image loading over it, and JPEG-only browsers get a progressive JPEG (mozjpeg-optimized), with the image’s dominant color as the background before the JPEG even starts loading.