CSS Testing with PhantomCSS, PhantomJS, CasperJS and Grunt
phase2technology.com
phase2technology.com
Might be your only option when testing non DOM parts of a webpage (such as canvas based) from the outside (e.g selenium or casper.js for example)
Last I checked (not too long ago), Phantom didn't support flexbox properly, and I'm heavily invested in it now. Hopefully I can find something that works.