text: /**
   * @license
   * Copyright 2013 The Emscripten Authors
   * SPDX-License-Identifier: MIT
   */
  
  function headlessCanvas() {
    var ret = {
      headless: true,
      getContext: function(which) {
        switch (which) {
          case 'webgl':
          case 'experimental-webgl': {
            return {
              /* ClearBufferMask */
              DEPTH_BUFFER_BIT               : 0x00000100,
              STENCIL_BUFFER_BIT             : 0x00000400,
              COLOR_BUFFER_BIT               : 0x00004000,
              
              /* BeginMode */
              POINTS                         : 0x0000,
              LINES                          : 0x0001,
              LINE_LOOP                      : 0x0002,
              LINE_STRIP                     : 0x0003,
              TRIANGLES                      : 0x0004,
              TRIANGLE_STRIP                 : 0x0005,
              TRIANGLE_FAN                   : 0x0006,
              
              /* AlphaFunction (not supported in ES20) */
              /*      NEVER */
              /*      LESS */
              /*      EQUAL */
              /*      LEQUAL */
              /*      GREATER */
              /*      NOTEQUAL */
              /*      GEQUAL */
              /*      ALWAYS */
              
              /* BlendingFactorDest */
              ZERO                           : 0,
              ONE                            : 1,
              SRC_COLOR                      : 0x0300,
              ONE_MINUS_SRC_COLOR            : 0x0301,
              SRC_ALPHA                      : 0x0302,
              ONE_MINUS_SRC_ALPHA            : 0x0303,
              DST_ALPHA                      : 0x0304,
              ONE_MINUS_DST_ALPHA            : 0x0305,
              
              /* BlendingFactorSrc */
              /*      ZERO */
              /*      ONE */
              DST_COLOR                      : 0x0306,
              ONE_MINUS_DST_COLOR            : 0x0307,
              SRC_ALPHA_SATURATE             : 0x0308,
              /*      SRC_ALPHA */
              /*      ONE_MINUS_SRC_ALPHA */
              /*      DST_ALPHA */
              /*      ONE_MINUS_DST_ALPHA */
              
              /* BlendEquationSeparate */
              FUNC_ADD                       : 0x8006,
              BLEND_EQUATION                 : 0x8009,
              BLEND_EQUATION_RGB             : 0x8009,   /* same as BLEND_EQUATION */
              BLEND_EQUATION_ALPHA           : 0x883D,
              
              /* BlendSubtract */
              FUNC_SUBTRACT                  : 0x800A,
              FUNC_REVERSE_SUBTRACT          : 0x800B,
              
              /* Separate Blend Functions */
              BLEND_DST_RGB                  : 0x80C8,
              BLEND_SRC_RGB                  : 0x80C9,
              BLEND_DST_ALPHA                : 0x80CA,
              BLEND_SRC_ALPHA                : 0x80CB,
              CONSTANT_COLOR                 : 0x8001,
              ONE_MINUS_CONSTANT_COLOR       : 0x8002,
              CONSTANT_ALPHA                 : 0x8003,
              ONE_MINUS_CONSTANT_ALPHA       : 0x8004,
              BLEND_COLOR                    : 0x8005,
              
              /* Buffer Objects */
              ARRAY_BUFFER                   : 0x8892,
              ELEMENT_ARRAY_BUFFER           : 0x8893,
              ARRAY_BUFFER_BINDING           : 0x8894,
              ELEMENT_ARRAY_BUFFER_BINDING   : 0x8895,
              
              STREAM_DRAW                    : 0x88E0,
              STATIC_DRAW                    : 0x88E4,
              DYNAMIC_DRAW                   : 0x88E8,
              
              BUFFER_SIZE                    : 0x8764,
              BUFFER_USAGE                   : 0x8765,
              
              CURRENT_VERTEX_ATTRIB          : 0x8626,
              
              /* CullFaceMode */
              FRONT                          : 0x0404,
              BACK                           : 0x0405,
              FRONT_AND_BACK                 : 0x0408,
              
              /* DepthFunction */
              /*      NEVER */
              /*      LESS */
              /*      EQUAL */
              /*      LEQUAL */
              /*      GREATER */
              /*      NOTEQUAL */
              /*      GEQUAL */
              /*      ALWAYS */
              
              /* EnableCap */
              /* TEXTURE_2D */
              CULL_FACE                      : 0x0B44,
              BLEND                          : 0x0BE2,
              DITHER                         : 0x0BD0,
              STENCIL_TEST                   : 0x0B90,
              DEPTH_TEST                     : 0x0B71,
              SCISSOR_TEST                   : 0x0C11,
              POLYGON_OFFSET_FILL            : 0x8037,
              SAMPLE_ALPHA_TO_COVERAGE       : 0x809E,
              SAMPLE_COVERAGE                : 0x80A0,
              
              /* ErrorCode */
              NO_ERROR                       : 0,
              INVALID_ENUM                   : 0x0500,
              INVALID_VALUE                  : 0x0501,
              INVALID_OPERATION              : 0x0502,
              OUT_OF_MEMORY                  : 0x0505,
              
              /* FrontFaceDirection */
              CW                             : 0x0900,
              CCW                            : 0x0901,
              
              /* GetPName */
              LINE_WIDTH                     : 0x0B21,
              ALIASED_POINT_SIZE_RANGE       : 0x846D,
              ALIASED_LINE_WIDTH_RANGE       : 0x846E,
              CULL_FACE_MODE                 : 0x0B45,
              FRONT_FACE                     : 0x0B46,
              DEPTH_RANGE                    : 0x0B70,
              DEPTH_WRITEMASK                : 0x0B72,
              DEPTH_CLEAR_VALUE              : 0x0B73,
              DEPTH_FUNC                     : 0x0B74,
              STENCIL_CLEAR_VALUE            : 0x0B91,
              STENCIL_FUNC                   : 0x0B92,
              STENCIL_FAIL                   : 0x0B94,
              STENCIL_PASS_DEPTH_FAIL        : 0x0B95,
              STENCIL_PASS_DEPTH_PASS        : 0x0B96,
              STENCIL_REF                    : 0x0B97,
              STENCIL_VALUE_MASK             : 0x0B93,
              STENCIL_WRITEMASK              : 0x0B98,
              STENCIL_BACK_FUNC              : 0x8800,
              STENCIL_BACK_FAIL              : 0x8801,
              STENCIL_BACK_PASS_DEPTH_FAIL   : 0x8802,
              STENCIL_BACK_PASS_DEPTH_PASS   : 0x8803,
              STENCIL_BACK_REF               : 0x8CA3,
              STENCIL_BACK_VALUE_MASK        : 0x8CA4,
              STENCIL_BACK_WRITEMASK         : 0x8CA5,
              VIEWPORT                       : 0x0BA2,
              SCISSOR_BOX                    : 0x0C10,
              /*      SCISSOR_TEST */
              COLOR_CLEAR_VALUE              : 0x0C22,
              COLOR_WRITEMASK                : 0x0C23,
              UNPACK_ALIGNMENT               : 0x0CF5,
              PACK_ALIGNMENT                 : 0x0D05,
              MAX_TEXTURE_SIZE               : 0x0D33,
              MAX_VIEWPORT_DIMS              : 0x0D3A,
              SUBPIXEL_BITS                  : 0x0D50,
              RED_BITS                       : 0x0D52,
              GREEN_BITS                     : 0x0D53,
              BLUE_BITS                      : 0x0D54,
              ALPHA_BITS                     : 0x0D55,
              DEPTH_BITS                     : 0x0D56,
              STENCIL_BITS                   : 0x0D57,
              POLYGON_OFFSET_UNITS           : 0x2A00,
              /*      POLYGON_OFFSET_FILL */
              POLYGON_OFFSET_FACTOR          : 0x8038,
              TEXTURE_BINDING_2D             : 0x8069,
              SAMPLE_BUFFERS                 : 0x80A8,
              SAMPLES                        : 0x80A9,
              SAMPLE_COVERAGE_VALUE          : 0x80AA,
              SAMPLE_COVERAGE_INVERT         : 0x80AB,
              
              /* GetTextureParameter */
              /*      TEXTURE_MAG_FILTER */
              /*      TEXTURE_MIN_FILTER */
              /*      TEXTURE_WRAP_S */
              /*      TEXTURE_WRAP_T */
              
              COMPRESSED_TEXTURE_FORMATS     : 0x86A3,
              
              /* HintMode */
              DONT_CARE                      : 0x1100,
              FASTEST                        : 0x1101,
              NICEST                         : 0x1102,
              
              /* HintTarget */
              GENERATE_MIPMAP_HINT            : 0x8192,
              
              /* DataType */
              BYTE                           : 0x1400,
              UNSIGNED_BYTE                  : 0x1401,
              SHORT                          : 0x1402,
              UNSIGNED_SHORT                 : 0x1403,
              INT                            : 0x1404,
              UNSIGNED_INT                   : 0x1405,
              FLOAT                          : 0x1406,
              
              /* PixelFormat */
              DEPTH_COMPONENT                : 0x1902,
              ALPHA                          : 0x1906,
              RGB                            : 0x1907,
              RGBA                           : 0x1908,
              LUMINANCE                      : 0x1909,
              LUMINANCE_ALPHA                : 0x190A,
              
              /* PixelType */
              /*      UNSIGNED_BYTE */
              UNSIGNED_SHORT_4_4_4_4         : 0x8033,
              UNSIGNED_SHORT_5_5_5_1         : 0x8034,
              UNSIGNED_SHORT_5_6_5           : 0x8363,
              
              /* Shaders */
              FRAGMENT_SHADER                  : 0x8B30,
              VERTEX_SHADER                    : 0x8B31,
              MAX_VERTEX_ATTRIBS               : 0x8869,
              MAX_VERTEX_UNIFORM_VECTORS       : 0x8DFB,
              MAX_VARYING_VECTORS              : 0x8DFC,
              MAX_COMBINED_TEXTURE_IMAGE_UNITS : 0x8B4D,
              MAX_VERTEX_TEXTURE_IMAGE_UNITS   : 0x8B4C,
              MAX_TEXTURE_IMAGE_UNITS          : 0x8872,
              MAX_FRAGMENT_UNIFORM_VECTORS     : 0x8DFD,
              SHADER_TYPE                      : 0x8B4F,
              DELETE_STATUS                    : 0x8B80,
              LINK_STATUS                      : 0x8B82,
              VALIDATE_STATUS                  : 0x8B83,
              ATTACHED_SHADERS                 : 0x8B85,
              ACTIVE_UNIFORMS                  : 0x8B86,
              ACTIVE_ATTRIBUTES                : 0x8B89,
              SHADING_LANGUAGE_VERSION         : 0x8B8C,
              CURRENT_PROGRAM                  : 0x8B8D,
              
              /* StencilFunction */
              NEVER                          : 0x0200,
              LESS                           : 0x0201,
              EQUAL                          : 0x0202,
              LEQUAL                         : 0x0203,
              GREATER                        : 0x0204,
              NOTEQUAL                       : 0x0205,
              GEQUAL                         : 0x0206,
              ALWAYS                         : 0x0207,
              
              /* StencilOp */
              /*      ZERO */
              KEEP                           : 0x1E00,
              REPLACE                        : 0x1E01,
              INCR                           : 0x1E02,
              DECR                           : 0x1E03,
              INVERT                         : 0x150A,
              INCR_WRAP                      : 0x8507,
              DECR_WRAP                      : 0x8508,
              
              /* StringName */
              VENDOR                         : 0x1F00,
              RENDERER                       : 0x1F01,
              VERSION                        : 0x1F02,
              
              /* TextureMagFilter */
              NEAREST                        : 0x2600,
              LINEAR                         : 0x2601,
              
              /* TextureMinFilter */
              /*      NEAREST */
              /*      LINEAR */
              NEAREST_MIPMAP_NEAREST         : 0x2700,
              LINEAR_MIPMAP_NEAREST          : 0x2701,
              NEAREST_MIPMAP_LINEAR          : 0x2702,
              LINEAR_MIPMAP_LINEAR           : 0x2703,
              
              /* TextureParameterName */
              TEXTURE_MAG_FILTER             : 0x2800,
              TEXTURE_MIN_FILTER             : 0x2801,
              TEXTURE_WRAP_S                 : 0x2802,
              TEXTURE_WRAP_T                 : 0x2803,
              
              /* TextureTarget */
              TEXTURE_2D                     : 0x0DE1,
              TEXTURE                        : 0x1702,
              
              TEXTURE_CUBE_MAP               : 0x8513,
              TEXTURE_BINDING_CUBE_MAP       : 0x8514,
              TEXTURE_CUBE_MAP_POSITIVE_X    : 0x8515,
              TEXTURE_CUBE_MAP_NEGATIVE_X    : 0x8516,
              TEXTURE_CUBE_MAP_POSITIVE_Y    : 0x8517,
              TEXTURE_CUBE_MAP_NEGATIVE_Y    : 0x8518,
              TEXTURE_CUBE_MAP_POSITIVE_Z    : 0x8519,
              TEXTURE_CUBE_MAP_NEGATIVE_Z    : 0x851A,
              MAX_CUBE_MAP_TEXTURE_SIZE      : 0x851C,
              
              /* TextureUnit */
              TEXTURE0                       : 0x84C0,
              TEXTURE1                       : 0x84C1,
              TEXTURE2                       : 0x84C2,
              TEXTURE3                       : 0x84C3,
              TEXTURE4                       : 0x84C4,
              TEXTURE5                       : 0x84C5,
              TEXTURE6                       : 0x84C6,
              TEXTURE7                       : 0x84C7,
              TEXTURE8                       : 0x84C8,
              TEXTURE9                       : 0x84C9,
              TEXTURE10                      : 0x84CA,
              TEXTURE11                      : 0x84CB,
              TEXTURE12                      : 0x84CC,
              TEXTURE13                      : 0x84CD,
              TEXTURE14                      : 0x84CE,
              TEXTURE15                      : 0x84CF,
              TEXTURE16                      : 0x84D0,
              TEXTURE17                      : 0x84D1,
              TEXTURE18                      : 0x84D2,
              TEXTURE19                      : 0x84D3,
              TEXTURE20                      : 0x84D4,
              TEXTURE21                      : 0x84D5,
              TEXTURE22                      : 0x84D6,
              TEXTURE23                      : 0x84D7,
              TEXTURE24                      : 0x84D8,
              TEXTURE25                      : 0x84D9,
              TEXTURE26                      : 0x84DA,
              TEXTURE27                      : 0x84DB,
              TEXTURE28                      : 0x84DC,
              TEXTURE29                      : 0x84DD,
              TEXTURE30                      : 0x84DE,
              TEXTURE31                      : 0x84DF,
              ACTIVE_TEXTURE                 : 0x84E0,
              
              /* TextureWrapMode */
              REPEAT                         : 0x2901,
              CLAMP_TO_EDGE                  : 0x812F,
              MIRRORED_REPEAT                : 0x8370,
              
              /* Uniform Types */
              FLOAT_VEC2                     : 0x8B50,
              FLOAT_VEC3                     : 0x8B51,
              FLOAT_VEC4                     : 0x8B52,
              INT_VEC2                       : 0x8B53,
              INT_VEC3                       : 0x8B54,
              INT_VEC4                       : 0x8B55,
              BOOL                           : 0x8B56,
              BOOL_VEC2                      : 0x8B57,
              BOOL_VEC3                      : 0x8B58,
              BOOL_VEC4                      : 0x8B59,
              FLOAT_MAT2                     : 0x8B5A,
              FLOAT_MAT3                     : 0x8B5B,
              FLOAT_MAT4                     : 0x8B5C,
              SAMPLER_2D                     : 0x8B5E,
              SAMPLER_CUBE                   : 0x8B60,
              
              /* Vertex Arrays */
              VERTEX_ATTRIB_ARRAY_ENABLED        : 0x8622,
              VERTEX_ATTRIB_ARRAY_SIZE           : 0x8623,
              VERTEX_ATTRIB_ARRAY_STRIDE         : 0x8624,
              VERTEX_ATTRIB_ARRAY_TYPE           : 0x8625,
              VERTEX_ATTRIB_ARRAY_NORMALIZED     : 0x886A,
              VERTEX_ATTRIB_ARRAY_POINTER        : 0x8645,
              VERTEX_ATTRIB_ARRAY_BUFFER_BINDING : 0x889F,
              
              /* Shader Source */
              COMPILE_STATUS                 : 0x8B81,
              
              /* Shader Precision-Specified Types */
              LOW_FLOAT                      : 0x8DF0,
              MEDIUM_FLOAT                   : 0x8DF1,
              HIGH_FLOAT                     : 0x8DF2,
              LOW_INT                        : 0x8DF3,
              MEDIUM_INT                     : 0x8DF4,
              HIGH_INT                       : 0x8DF5,
              
              /* Framebuffer Object. */
              FRAMEBUFFER                    : 0x8D40,
              RENDERBUFFER                   : 0x8D41,
              
              RGBA4                          : 0x8056,
              RGB5_A1                        : 0x8057,
              RGB565                         : 0x8D62,
              DEPTH_COMPONENT16              : 0x81A5,
              STENCIL_INDEX                  : 0x1901,
              STENCIL_INDEX8                 : 0x8D48,
              DEPTH_STENCIL                  : 0x84F9,
              
              RENDERBUFFER_WIDTH             : 0x8D42,
              RENDERBUFFER_HEIGHT            : 0x8D43,
              RENDERBUFFER_INTERNAL_FORMAT   : 0x8D44,
              RENDERBUFFER_RED_SIZE          : 0x8D50,
              RENDERBUFFER_GREEN_SIZE        : 0x8D51,
              RENDERBUFFER_BLUE_SIZE         : 0x8D52,
              RENDERBUFFER_ALPHA_SIZE        : 0x8D53,
              RENDERBUFFER_DEPTH_SIZE        : 0x8D54,
              RENDERBUFFER_STENCIL_SIZE      : 0x8D55,
              
              FRAMEBUFFER_ATTACHMENT_OBJECT_TYPE           : 0x8CD0,
              FRAMEBUFFER_ATTACHMENT_OBJECT_NAME           : 0x8CD1,
              FRAMEBUFFER_ATTACHMENT_TEXTURE_LEVEL         : 0x8CD2,
              FRAMEBUFFER_ATTACHMENT_TEXTURE_CUBE_MAP_FACE : 0x8CD3,
              
              COLOR_ATTACHMENT0              : 0x8CE0,
              DEPTH_ATTACHMENT               : 0x8D00,
              STENCIL_ATTACHMENT             : 0x8D20,
              DEPTH_STENCIL_ATTACHMENT       : 0x821A,
              
              NONE                           : 0,
              
              FRAMEBUFFER_COMPLETE                      : 0x8CD5,
              FRAMEBUFFER_INCOMPLETE_ATTACHMENT         : 0x8CD6,
              FRAMEBUFFER_INCOMPLETE_MISSING_ATTACHMENT : 0x8CD7,
              FRAMEBUFFER_INCOMPLETE_DIMENSIONS         : 0x8CD9,
              FRAMEBUFFER_UNSUPPORTED                   : 0x8CDD,
              
              FRAMEBUFFER_BINDING            : 0x8CA6,
              RENDERBUFFER_BINDING           : 0x8CA7,
              MAX_RENDERBUFFER_SIZE          : 0x84E8,
              
              INVALID_FRAMEBUFFER_OPERATION  : 0x0506,
              
              /* WebGL-specific enums */
              UNPACK_FLIP_Y_WEBGL            : 0x9240,
              UNPACK_PREMULTIPLY_ALPHA_WEBGL : 0x9241,
              CONTEXT_LOST_WEBGL             : 0x9242,
              UNPACK_COLORSPACE_CONVERSION_WEBGL : 0x9243,
              BROWSER_DEFAULT_WEBGL          : 0x9244,
  
              items: {},
              id: 0,
              getExtension: function() { return 1 },
              createBuffer: function() {
                var id = this.id++;
                this.items[id] = {
                  which: 'buffer',
                };
                return id;
              },
              deleteBuffer: function(){},
              bindBuffer: function(){},
              bufferData: function(){},
              getParameter: function(pname) {
                switch (pname) {
                  case /* GL_VENDOR                           */ 0x1F00: return 'FakeShellGLVendor';
                  case /* GL_RENDERER                         */ 0x1F01: return 'FakeShellGLRenderer';
                  case /* GL_VERSION                          */ 0x1F02: return '0.0.1';
                  case /* GL_MAX_TEXTURE_SIZE                 */ 0x0D33: return 16384;
                  case /* GL_MAX_CUBE_MAP_TEXTURE_SIZE        */ 0x851C: return 16384;
                  case /* GL_MAX_TEXTURE_MAX_ANISOTROPY_EXT   */ 0x84FF: return 16;
                  case /* GL_MAX_TEXTURE_IMAGE_UNITS_NV       */ 0x8872: return 16;
                  case /* GL_MAX_VERTEX_UNIFORM_VECTORS       */ 0x8DFB: return 4096;
                  case /* GL_MAX_FRAGMENT_UNIFORM_VECTORS     */ 0x8DFD: return 4096;
                  case /* GL_MAX_VARYING_VECTORS              */ 0x8DFC: return 32;
                  case /* GL_MAX_COMBINED_TEXTURE_IMAGE_UNITS */ 0x8B4D: return 32;
                  case /* GL_ARRAY_BUFFER_BINDING             */ 0x8894: return 0;
                  default: console.log('getParameter ' + pname + '?'); return 0;
                }
              },
              getSupportedExtensions: function() {
                return ["OES_texture_float", "OES_standard_derivatives", "EXT_texture_filter_anisotropic", "MOZ_EXT_texture_filter_anisotropic", "MOZ_WEBGL_lose_context", "MOZ_WEBGL_compressed_texture_s3tc", "MOZ_WEBGL_depth_texture"];
              },
              createShader: function(type) {
                var id = this.id++;
                this.items[id] = {
                  which: 'shader',
                  type: type,
                };
                return id;
              },
              getShaderParameter: function(shader, pname) {
                switch (pname) {
                  case /* GL_SHADER_TYPE    */ 0x8B4F: return this.items[shader].type;
                  case /* GL_COMPILE_STATUS */ 0x8B81: return true;
                  default: throw 'getShaderParameter ' + pname;
                }
              },
              shaderSource: function(){},
              compileShader: function(){},
              createProgram: function() {
                var id = this.id++;
                this.items[id] = {
                  which: 'program',
                  shaders: [],
                };
                return id;
              },
              attachShader: function(program, shader) {
                this.items[program].shaders.push(shader);
              },
              bindAttribLocation: function(){},
              linkProgram: function(){},
              getProgramParameter: function(program, pname) {
                switch (pname) {
                  case /* LINK_STATUS     */ 0x8B82: return true;
                  case /* ACTIVE_UNIFORMS */ 0x8B86: return 4;
                  default: throw 'getProgramParameter ' + pname;
                }
              },
              deleteShader: function(){},
              deleteProgram: function(){},
              viewport: function(){},
              clearColor: function(){},
              clearDepth: function(){},
              depthFunc: function(){},
              enable: function(){},
              disable: function(){},
              frontFace: function(){},
              cullFace: function(){},
              activeTexture: function(){},
              createTexture: function() {
                var id = this.id++;
                this.items[id] = {
                  which: 'texture',
                };
                return id;
              },
              deleteTexture: function(){},
              boundTextures: {},
              bindTexture: function(target, texture) {
                this.boundTextures[target] = texture;
              },
              texParameteri: function(){},
              pixelStorei: function(){},
              texImage2D: function(){},
              compressedTexImage2D: function(){},
              useProgram: function(){},
              getUniformLocation: function() {
                return null;
              },
              getActiveUniform: function(program, index) {
                return {
                  size: 1,
                  type: /* INT_VEC3 */ 0x8B54,
                  name: 'activeUniform' + index,
                };
              },
              clear: function(){},
              uniform4fv: function(){},
              uniform1i: function(){},
              getAttribLocation: function() { return 1 },
              vertexAttribPointer: function(){},
              enableVertexAttribArray: function(){},
              disableVertexAttribArray: function(){},
              drawElements: function(){},
              drawArrays: function(){},
              depthMask: function(){},
              depthRange: function(){},
              bufferSubData: function(){},
              blendFunc: function(){},
              createFramebuffer: function() {
                var id = this.id++;
                this.items[id] = {
                  which: 'framebuffer',
                  shaders: [],
                };
                return id;
              },
              bindFramebuffer: function(){},
              framebufferTexture2D: function(){},
              checkFramebufferStatus: function() {
                return /* FRAMEBUFFER_COMPLETE */ 0x8CD5;
              },
              createRenderbuffer: function() {
                var id = this.id++;
                this.items[id] = {
                  which: 'renderbuffer',
                  shaders: [],
                };
                return id;
              },
              bindRenderbuffer: function(){},
              renderbufferStorage: function(){},
              framebufferRenderbuffer: function(){},
              scissor: function(){},
              colorMask: function(){},
              lineWidth: function(){},
              vertexAttrib4fv: function(){},
            };
          }
          case '2d': {
            return {
              drawImage: function(){},
              getImageData: function(x, y, w, h) {
                return {
                  width: w,
                  height: h,
                  data: new Uint8ClampedArray(w*h),
                };
              },
              save: function(){},
              restore: function(){},
              fillRect: function(){},
              measureText: function() { return 10 },
              fillText: function(){},
            };
          }
          default: throw 'canvas.getContext: ' + which;
        }
      },
      requestPointerLock: function() {
        document.pointerLockElement = document.getElementById('canvas');
        window.setTimeout(function() {
          document.callEventListeners('pointerlockchange');
        });
      },
      exitPointerLock: function(){},
      style: {
        setProperty: function() {},
        removeProperty: function() {},
      },
      eventListeners: {},
      addEventListener: function(){},
      removeEventListener: function(){},
      requestFullscreen: function() {
        document.fullscreenElement = document.getElementById('canvas');
        window.setTimeout(function() {
          document.callEventListeners('fullscreenchange');
        });
      },
      offsetTop: 0,
      offsetLeft: 0,
      // generics
      classList: {
        add: function(){},
        remove: function(){},
      },
      insertBefore: function(){},
    };
    ret.parentNode = ret;
    return ret;
  }
  
  