WebGLのデバッグ用拡張を使えば取得できるらしい。
<!doctype html>
<html>
<body>
<canvas id="canvas" width="0" height="0"></canvas>
<script type="text/JavaScript">
var canvas;
canvas = document.getElementById("canvas");
var gl = canvas.getContext("experimental-webgl");
var renderInfo = gl.getExtension("WEBGL_debug_renderer_info");
if (renderInfo != null) {
var vendor = gl.getParameter(renderInfo.UNMASKED_VENDOR_WEBGL);
var renderer = gl.getParameter(renderInfo.UNMASKED_RENDERER_WEBGL);
document.write(vendor + "<br>");
document.write(renderer + "<br>");
}
</script>
</body>
</html>
出力例)
ATI Technologies Inc.
AMD Radeon R9 M295X OpenGL Engine