If sites do not display properly in Opera, this site analysis may hint at what the problem is. Please note: this is testing against Opera 6.05's JavaScript support. Opera 7's support is much more extensive and this script will be updated when I have a moment.
Bookmarklet for using this sitetest: Test page. (Right-click this link and select add to bookmarks.)
Version that opens new window: Test page
New: bookmarklet for listing external scripts on page: List external scripts (Right-click this link and select add to bookmarks. This works in browsers with good DOM support, O07, Moz etc.)
What's this? This site will simply scan the JavaScript on a page and external script files linked to for commands known not to work with the current release of the Opera browser. It is not a validation service. Tools such as W3C's validator are recommended.
Note that often there are good reasons why Opera Software has chosen not to implement features. Since this is not a validation service, those reasons will not be discussed here. Please don't send Opera Software bug reports and complaints based on this test, they already know every detail about the lack of support for the functions this page tests for.