How much should I rely on Geometry shaders in WebGL? Geometry shaders appear to have been introduced in 3.2, which makes me wonder how common 3.2-enabled cards are, including support for Geometry shaders within WebGL contexts.

Will I be cutting out a large portion of viewers by requiring the use of a geometry shader?
WebGL doesn't even currently support geometry shaders, so to directly answer the question: 100% of all users.