How to run IE7 and IE8 on IE9

I once downgraded IE9 to IE8 for testing a web page whether it can work well on IE8. But if you set auto upgrade you will find IE return IE9 soon. And using IE8 on WIN7 gives you hell, too.
Can we run different versions IE on one browser? Yes, it is IE9. Finally I found it, because I firmly believe Microsoft is a big enough and powerful enough company and should provide this developer-friendly functionality which I even can think of. In fact, I once raised similar issue as early as IE7 was launched and wondered why IE7 didn't support such functionality to facilitate testing IE6 compatibility.
Below are the steps how to switch between several IE versions:
1. Press F12 to open developer tools or use menu

2. Change Browser Mode and Document Mode

What are Browser Mode and Document Mode? You can refer to here. 
And I also just heard this functionality was provided from IE8.... Is that because during this time I maintain a very old EJB2 project which only was tested on IE6.

So far I find IE9 can do all what Firefox do, and it might do even better. IE9, Firefox and Chrome, three pillars age?

How to forbid Speed up browsing by disable add-ons

You may often see this popup when every times you open you IE9 browser to surf.

It is really disappointing, at least I feel so. But how to forbid it? Easy, there are two methods:
A. Forbid this notification directly, I call it direct method;
B. Don't let it appear easily, I call it indirect method.
They are explained below:

A. Direct method
1. Win Key + r and type in regedit, and Alt + y to open window's Registry Edit

2. Go in \HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion\Policies

3. Right click Policies node and create a new key, name it Ext

4. Click Ext just created, on right side empty place right click and select DWORD(32-bit)Value

5. Name it DisableAddonLoadTimePerformanceNotifications

6. Double click this new created item, set value to 1, by default it's 0

7. Click OK, exit registry edit, and restart your IE9 browser.
You will find this notification is gone. But how to let it back? ... This is another question! :)

B. Indirect method
This notification is for telling you the add-ons increase the time to start the browser,open a new tab or navigate to websites and is only displayed when this time exceeds a certain time. We can set up a very long time so that the start time will not exceed it.
1. Display Command bar of your IE9

 Then you can see

2. Open Toolbars --> Disable add-ons

3. Set up a very long limit time, select the longest time option, 10.0 seconds

4. Click Done. Start a new tab, this notification should be gone.

5. If you want to let it come back very easy. Just set a short time.