This is handled on a site by site basis. If you are testing your site. This is a link to an overview of the code that needs to be changed in processing.js.
http://asalga.wordpress.com/2010/04/15/webgl-anti-aliasing-in-processingjs/ If you need to test on another persons site you could try downloading the source for their site then testing it that way.
A good easy way to copy a website is to use httrack. www.httrack.com It's a website copier that also has a firefox extension called spiderzilla.