A lightweight version of SwiftShader was licensed for Flash and AIR (http://transgaming.com/news/transgaming-s-swiftshader-3d-tec...).

That said, SwiftShader also features an OpenGL ES 2.0 API, making it highly suitable for WebGL implementations as well. In fact Google is currently in the process of integrating SwiftShader support into Chrome (http://codereview.chromium.org/8480015/).