AutoHotKey GUI with embedded HTML and transparent background
<p>I'm trying to make an AutoHotKey script to display a transparent overlay from an HTML file using the following code:</p>

<pre><code>SendMode Input  ; Recommended for new scripts due to its superior speed and reliability.
SetWorkingDir %A_ScriptDir%  ; Ensures a consistent starting directory.
#NoEnv  ; Recommended for performance and compatibility with future AutoHotkey releases.
#Warn  ; Enable warnings to assist with detecting common errors.
#SingleInstance, force
Menu Tray, tip, %A_ScriptName% ; Custom traytip
OverlayVisible := 0

; ---------------------- CUSTOMIZE BELOW HERE ----------------------

PositionX      := 31     ; x position of top left corner of overlay
PositionY      := 423    ; y position of top left corner of overlay

F1::
    Gosub Sub_BuildHTML
    Gosub Sub_ToggleOverlay
return

Sub_ToggleOverlay:
    if !OverlayVisible {
        Gosub Sub_ShowOverlay
    } else {
        Gosub Sub_HideOverlay
    }
    OverlayVisible := !OverlayVisible
return

; Creates and shows the GUI
Sub_ShowOverlay:
    Gui GUI_Overlay:New, +ToolWindow  +LastFound +AlwaysOnTop -Caption -Border +hwndGUI_Overlay_hwnd

    Gui Margin, 0,0  
    Gui Add, ActiveX, w400 h225 vWB BackGroundTrans, Shell.Explorer
    URL := "file:///" . A_ScriptDir . "/overlay.html"
    WB.Navigate(URL)

    WinSet Transparent, 255
    ;WinSet TransColor, White 0
    Gui Show, Hide, Overlay

    WinMove PositionX, PositionY
    Gui GUI_Overlay:Show, NoActivate
return

Sub_HideOverlay:
    Gui GUI_Overlay:Destroy
return

Sub_BuildHTML:
    FileDelete, overlay.html
    FileAppend,
    (
&lt;!DOCTYPE html&gt;
&lt;html&gt;
  &lt;meta charset="utf-8" http-equiv="X-UA-Compatible" content="IE=edge"/&gt;
  &lt;head&gt;
    &lt;style type="text/css"&gt;
    body {
      background-color: transparent;
      overflow:hidden;
    }
    #header1 {
      color:blue;
    }
    &lt;/style&gt;
  &lt;/head&gt;
  &lt;body&gt;
    &lt;div id="header1"&gt;TEST&lt;/div&gt;
    &lt;img src="http://i.stack.imgur.com/FBZq5.png" /&gt;
  &lt;/body&gt;
&lt;/html&gt;
), overlay.html
return
</code></pre>

<p>Everything except the transparency is working great. I can use WinSet Transparent to make the entire control transparent but what I want is for just the edges of the overlay to fade out to transparency so that the actual content of the HTML is visible. </p>

<p>I created a background image for the HTML with the desired fade to transparent but because the background of the HTML is still white it is just a fade to white instead. </p>

<p>Seems to be an issue with IE not wanting to render a transparent background? Is there any way to fix this so that the background is really transparent or is there some other workaround possible?</p>