You (or your clients) can use Trello on IE8 after installing the Google Chrome Frame.
In fact, if you attempt to use Trello with IE8, you'll get a message that gives you that advice:
Your browser is not supported.
Trello supports Chrome 9 and above, Firefox 4 and above, Safari 5.0.5
and above, and Internet Explorer 9 and above.
This could be add-ons issue. Clear cache. Try with InPrivate window. Try to update Internet Explorer.
From Microsoft Support:
When you experience problems playing videos in Internet Explorer, these problems may be caused by a number of different issues. Most video playback uses Adobe Flash, Microsoft Silverlight, or Java, and these platforms require ...
For these cases you should always consider indicating:
Each operating system and its version
Each browser and its version
Once you have gathered this informarion check Trello support for those platforms at trello help
If everything should be fine, try to reproduce the problem in other computers having the same platforms.
If still if you have problems, ...
From: "John Greet (GitHub Staff)"
Are you signed in to GitHub in Internet Explorer?
SSH access requires a GitHub account with a public key associated with it, even
for public repositories, so we don't show the SSH remote in any browser if you
are logged out.