Custom HTTP error page is not displayed in Internet Explorer

Solution 1:

This is an IE feature. When an HTTP error page retrieved from the server is smaller than 512 bytes, then IE will by default show a "Friendly" error page like the one you're facing, which is configureable by Tools > Internet Options > Advanced > Uncheck "Show Friendly Error Message" in the browser. Other (real) browsers does not have this feature.

Making your HTTP error page a little larger than 512 bytes should workaround this IE feature. You could add some extra meta headers, add some whitespace to indent code, add some more semantic markup following your site's standard layout, add a large HTML comment, etc.