I used the 1080p public domain tractor sample from
xiph.org/.../y4m and scaled it down for web use while keeping the lossless y4m format. The video was then generated using this command line:
../vpxenc -o vp9l.webm --codec=vp9 --lossless=1 input.y4m
The master branch of the git repo as of Nov. 15 2014 was used. Though the bitstream was already finalized, the encoder may have been improved in the meantime. Consequently up-to-date encoding results may vary in file size and quality.