Right now all major browsers (and many encoders) support progressive jpegs
https://en.wikipedia.org/wiki/Comparison_of_layout_engines_%28graphics%29#JPEG_support

Regarding rendering performance progressive jpegs are absolutely recommended.
http://blog.patrickmeenan.com/2013/06/progressive-jpegs-ftw.html

I would suggest changing the paragraph in level-2 to using progressive jpeg by default when the encoder supports it.
https://github.com/riapi/riapi/blob/master/level-2.md#jpeg-encoding-customization