Why not use Internet Explorer 9’s Developer Tools for testing IE8 & IE7? Here’s why.

Twice last week this issue has come up so I’m providing some examples and reasoning as to why you should not even bother using Internet Explorer 9 for testing websites for how they would look in Internet Explorer 8 and Internet Explorer 7.

Background
IE has traditionally not been developer friendly out of the box. In IE9, that changed and Microsoft provided Compatibility Modes within Developer Tools (F12).

Months back, working with a couple devs, I was told this is how they test with IE8. I thought “wow, great that Microsoft is trying to help developers.” I never looked into this feature before, as I had always just used VMs for testing, long before IE9. So when IE9 came out, I continued that patter. (Microsoft provides free virtual machine images for testing.) However, my distrust for them overpowered me and I decided to run some tests.

The first test I ran was Acid3 http://acid3.acidtests.org/ Viewing the screenshots below, you will see the immediate differences I saw: A different score and slightly different rendering (eg “FAIL” in the corner.) Continue reading →