You just have to add:
html, body {
margin: 0;
}
You set the canvas width to 100% of the window size but forget to subtract the margin of body.
Question
In http://onlyabutton.com/, when you press the left and right arrow keys, the window shakes a bit, most likely because of canvas dimensions. How do I fix this? Help would be appreciated!
Solution
You just have to add:
html, body {
margin: 0;
}
You set the canvas width to 100% of the window size but forget to subtract the margin of body.