Internet Explorer as is only to be expected does not handle fieldset tags correctly when applying a background. This isn’t all that surprising given all the other things that the really antiquated versions of IE (7 and earlier) do incorrectly when you compare those browsers with modern alternatives.
In this case what is happening is that [...]

One of the biggest problems with Internet Explorer is that it is integrated far to much into the operating system. This has several impacts.

You can’t uninstall it from your computer if you have decided to use a better browser
It only runs on Windows and not on any of the other platforms where the web can [...]

Back in the early days of stylesheets support for the various commands varied somewhat between browsers. Quite a number of browsers also had quirks where they misinterpreted stylesheet commands either ignoring an invalid character and processing a command that ought to be ignored or by not recognising a valid character and so not processing a [...]