If the requested type is image/jpeg or image/webp,then the second
argument,if it is between 0.0 and 1.0,is treated as indicating image
quality; if the second argument is anything else,the default value
for image quality is used. Other arguments are ignored.
但我无法找到告诉我默认值实际是什么的任何地方.
解决方法
The second argument,if it is a number in the range 0.0 to 1.0 inclusive,must be treated as the desired quality level. If it is not a number or is outside that range,the user agent must use its default value,as if the argument had been omitted.
编辑：根据one user,Firefox的默认值为0.92.
You can specify the JPEG quality as the second parameter to the toDataURL function. The default quality in Firefox is 0.92 (92%).
根据这个webkit bug report Chrome使用相同的.
…Adds a libjpeg-based image encoder for Skia bitmaps. Default encoding quality is 92 to match Mozilla…
