var pattern = ctx.createPattern(img, null);
null is converted into the string "null", so per <http://www.whatwg.org/specs/web-apps/current-work/multipage/the-canvas-element.html#dom-context-2d-createpattern> "If an unrecognized value is given, then the user agent must throw a SyntaxError exception.", this should throw.
I changed the WHATWG spec on this, since the browsers didn't align. r8187 :-(
I blame the W3C fork for this, it caused confusion, as usual.
Adding "needs-testcase" keyword; http://w3c-test.org/testrunner/2dcontext/fill-and-stroke-styles/2d.pattern.repeat.null.html needs to be updated. Perhaps that amounts to backing out https://github.com/w3c/web-platform-tests/commit/398c348ab9827a450a2e6a3e15e9676ee8b6892a ?