diff --git a/spec/Makefile b/spec/Makefile
index 193a83921e..7c23be86dc 100644
--- a/spec/Makefile
+++ b/spec/Makefile
@@ -3,7 +3,7 @@ all: index.html webgpu.idl
 index.html: index.bs
 	bikeshed -f spec index.bs
 
-webgpu.idl: index.bs
+webgpu.idl: index.bs extract-idl.py
 	python extract-idl.py index.bs > webgpu.idl
 
 online:
diff --git a/spec/extract-idl.py b/spec/extract-idl.py
index 29429e45ec..a0ea13dca3 100755
--- a/spec/extract-idl.py
+++ b/spec/extract-idl.py
@@ -19,7 +19,7 @@
 
 // **** This file is auto-generated. Do not edit. ****
 
-"""
+""".lstrip()
 
 inputfilename = sys.argv[1]
 inputfile = open(inputfilename)
@@ -28,15 +28,17 @@
 idlStart = re.compile("\<script .*type=[\'\"]?idl")
 idlStop = re.compile("\</script\>")
 
+idlLineList = []
 for line in inputfile:
     line = line.rstrip()
     if idlStart.search(line) != None:
         recording = True
     elif idlStop.search(line) != None:
-        idlList.append("\n")
         recording = False
+        idlList.append("\n".join(idlLineList))
+        idlLineList = []
     elif recording:
-        idlList.append(line)
+        idlLineList.append(line)
 
 headerTemplate = Template(HEADER)
-print headerTemplate.substitute(YEAR=date.today().year) + "\n".join(idlList)
+print headerTemplate.substitute(YEAR=date.today().year) + "\n\n\n".join(idlList)
diff --git a/spec/index.bs b/spec/index.bs
index e29382ba8e..cfa8f169b8 100644
--- a/spec/index.bs
+++ b/spec/index.bs
@@ -42,8 +42,10 @@ dictionary GPUExtent3D {
     required u32 depth;
 };
 
+typedef sequence<any> GPUMappedBuffer;  // [GPUBuffer, ArrayBuffer]
+
 interface GPUObjectBase {
-    DOMString? label;
+    attribute DOMString? label;
 };
 
 dictionary GPUObjectDescriptorBase {
@@ -58,15 +60,15 @@ Buffers {#buffers}
 typedef u32 GPUBufferUsageFlags;
 
 interface GPUBufferUsage {
-    const u32 NONE = 0;
-    const u32 MAP_READ = 1;
-    const u32 MAP_WRITE = 2;
-    const u32 TRANSFER_SRC = 4;
-    const u32 TRANSFER_DST = 8;
-    const u32 INDEX = 16;
-    const u32 VERTEX = 32;
-    const u32 UNIFORM = 64;
-    const u32 STORAGE = 128;
+    const u32 NONE         = 0x0000;
+    const u32 MAP_READ     = 0x0001;
+    const u32 MAP_WRITE    = 0x0002;
+    const u32 TRANSFER_SRC = 0x0004;
+    const u32 TRANSFER_DST = 0x0008;
+    const u32 INDEX        = 0x0010;
+    const u32 VERTEX       = 0x0020;
+    const u32 UNIFORM      = 0x0040;
+    const u32 STORAGE      = 0x0080;
 };
 
 dictionary GPUBufferDescriptor : GPUObjectDescriptorBase {
@@ -167,12 +169,12 @@ enum GPUTextureFormat {
 typedef u32 GPUTextureUsageFlags;
 
 interface GPUTextureUsage {
-    const u32 NONE = 0;
-    const u32 TRANSFER_SRC = 1;
-    const u32 TRANSFER_DST = 2;
-    const u32 SAMPLED = 4;
-    const u32 STORAGE = 8;
-    const u32 OUTPUT_ATTACHMENT = 16;
+    const u32 NONE              = 0x00;
+    const u32 TRANSFER_SRC      = 0x01;
+    const u32 TRANSFER_DST      = 0x02;
+    const u32 SAMPLED           = 0x04;
+    const u32 STORAGE           = 0x08;
+    const u32 OUTPUT_ATTACHMENT = 0x10;
 };
 
 dictionary GPUTextureDescriptor : GPUObjectDescriptorBase {
@@ -271,10 +273,10 @@ Binding and Layout {#binding}
 typedef u32 GPUShaderStageFlags;
 
 interface GPUShaderStageBit {
-    const u32 NONE = 0;
-    const u32 VERTEX = 1;
-    const u32 FRAGMENT = 2;
-    const u32 COMPUTE = 4;
+    const u32 NONE     = 0x0;
+    const u32 VERTEX   = 0x1;
+    const u32 FRAGMENT = 0x2;
+    const u32 COMPUTE  = 0x4;
 };
 
 enum GPUBindingType {
@@ -398,12 +400,12 @@ enum GPUBlendOperation {
 
 typedef u32 GPUColorWriteFlags;
 interface GPUColorWriteBits {
-    const u32 NONE = 0;
-    const u32 RED = 1;
-    const u32 GREEN = 2;
-    const u32 BLUE = 4;
-    const u32 ALPHA = 8;
-    const u32 ALL = 15;
+    const u32 NONE  = 0x0;
+    const u32 RED   = 0x1;
+    const u32 GREEN = 0x2;
+    const u32 BLUE  = 0x4;
+    const u32 ALPHA = 0x8;
+    const u32 ALL   = 0xF;
 };
 
 dictionary GPUBlendDescriptor {
@@ -417,7 +419,7 @@ dictionary GPUColorStateDescriptor {
 
     required GPUBlendDescriptor alphaBlend;
     required GPUBlendDescriptor colorBlend;
-    GPUColorWriteFlags writeMask = GPUColorWriteBits.ALL;
+    GPUColorWriteFlags writeMask = 0xF;  // GPUColorWriteBits.ALL
 };
 
 enum GPUStencilOperation {
@@ -647,7 +649,7 @@ dictionary GPURenderPassColorAttachmentDescriptor {
 
     required GPULoadOp loadOp;
     required GPUStoreOp storeOp;
-    GPUColor clearColor = {r: 0.0, g: 0.0, b: 0.0, a: 1.0};
+    GPUColor clearColor;  // defaults to {r: 0.0, g: 0.0, b: 0.0, a: 1.0}
 };
 
 dictionary GPURenderPassDepthStencilAttachmentDescriptor {
@@ -684,7 +686,7 @@ dictionary GPUTextureCopyView {
     required GPUTexture texture;
     u32 mipLevel = 0;
     u32 arrayLayer = 0;
-    GPUOrigin3D origin = {x: 0, y: 0, z: 0};
+    GPUOrigin3D origin;  // defaults to {x: 0, y: 0, z: 0}
 };
 
 interface GPUCommandBuffer : GPUObjectBase {
@@ -761,12 +763,12 @@ interface GPUCanvasContext {
     GPUSwapChain configureSwapChain(GPUSwapChainDescriptor descriptor);
 
     Promise<GPUTextureFormat> getSwapChainPreferredFormat(GPUDevice device);
-}
+};
 
 dictionary GPUSwapChainDescriptor : GPUObjectDescriptorBase {
     required GPUDevice device;
     required GPUTextureFormat format;
-    GPUTextureUsageFlags usage = GPUTextureUsage.OUTPUT_ATTACHMENT;
+    GPUTextureUsageFlags usage = 0x10;  // GPUTextureUsage.OUTPUT_ATTACHMENT
 };
 
 interface GPUSwapChain : GPUObjectBase {
@@ -793,8 +795,8 @@ interface GPUDevice : GPUObjectBase {
     readonly attribute GPUAdapter adapter;
 
     GPUBuffer createBuffer(GPUBufferDescriptor descriptor);
-    (GPUBuffer, ArrayBuffer) createBufferMapped(GPUBufferDescriptor descriptor);
-    Promise<(GPUBuffer, ArrayBuffer)> createBufferMappedAsync(GPUBufferDescriptor descriptor);
+    GPUMappedBuffer createBufferMapped(GPUBufferDescriptor descriptor);
+    Promise<GPUMappedBuffer> createBufferMappedAsync(GPUBufferDescriptor descriptor);
     GPUTexture createTexture(GPUTextureDescriptor descriptor);
     GPUSampler createSampler(GPUSamplerDescriptor descriptor);
 
@@ -912,8 +914,11 @@ dictionary GPUUncapturedErrorEventInit : EventInit {
     required GPUError error;
 };
 
-// TODO: is it possible to expose the EventTarget only on the main thread?
-partial interface GPUDevice : EventTarget {
+// TODO: EventTarget is actually an interface, not a mixin, but it probably should be a mixin.
+[Exposed=Window]
+GPUDevice includes EventTarget;
+
+partial interface GPUDevice {
     [Exposed=Window]
     attribute EventHandler onuncapturederror;
 };