I'd take a look at http://caniuse.com/#compare=chrome+51,safari+9.1; that shows what HTML5 constructs Chrome/Blink allow that Safari (and hence webkit) don't. Also, according to http://phantomjs.org/supported-web-standards.html,
Support for plugins (such as Flash) was dropped a long time ago. The primary reasons:
Pure headless (no X11) makes it impossible to have windowed plugin
Issues and bugs are hard to debug, due to the proprietary nature of
such plugins (binary blobs) The following features, due to the nature
of PhantomJS, are irrelevant:
WebGL would require an OpenGL-capable system. Since the goal of
PhantomJS is to become 100% headless and self-contained, this is not
acceptable. Using OpenGL emulation via Mesa can overcome the
limitation, but then the performance would degrade.
Video and Audio would require shipping a variety of different codecs.
CSS 3-D needs a perspective-correct implementation of texture mapping.
It can’t be implemented without a penalty in performance.
Each of the above feature may be supported in the future if the
technical challenges associated with the implementations are solved.
Until then, do not rely on those features.
So anything that requires Flash, for example, isn't going to be testable via phantomjs. Things that might involve window resizing isn't going to be testable, it appears.