A player size that matches the stream resolution exactly.
Prevents unnecessary blur from the filtering applied when resizing the video. Great for retro game streamers using integer scaling.
It's interesting to note that the Twitch player is pretty close to 1280x720 on a 1920x1080 screen, and pretty close to 1920x1080 on a 2560x1440 screen. Pretty close, but not quite there.
I've created a userstyle (a few lines of styling code that you can apply locally in your browser) that fixes this with no noticeable impact on the rest of the page, and makes 720p and 1080p streams crispy on monitors 1920 pixels wide and 2560 pixels wide respectively. You can find it here https://github.com/outfrost/usercss#twitch-player-resolution-fix and install it with a browser extension like Stylus.
That being said, I feel like Twitch should do this by default, as it seems like an obvious omission once you look at the "video stats" window in a stream.