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Review of NW.js (w/xvfb) powered by the Slant community.
While built for a different purpose (Node based desktop apps), NW.js works quite well for headless testing.
SpecsUpdate
Supported languagesJS, [third party modules:C#,C++Web Assembly]
Integrated DebuggerSDK version
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Pros

Pro Node integration
The integration of Node with the DOM in NW.js opens up a number of new options in how your headless testing workflow can be facilitated.
The distinct separation of JavaScript contexts, the introduction of a separate node context, and the ability to cross communicate, offers a lot of power and flexibility.

Pro NaCl support/integration
Access to the Native Client offers up more options in implementing your testing workflow.
Cons

Con xvfb Requirement (for now)
Headless is on the NW.js roadmap, but for now xvfb is necessary to get going. The community has done the work and you can find the details here.
Commonly Compared
NW.js (w/xvfb)
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