The Opera web browser is an alternative browser to the Netscape and MSIE browsers on the Windows platform. It is highly standards-based, and as a result has gained much respect. However, because many pages do not contain strictly standard content, you should quality-check your site on this browser to ensure its compliance. Using Opera as a reference browser can significantly improve the likelihood of your site continuing to work in the future as all browsers are likely to converge on the functionality embodied in the standards.
Details of what is supported in version 5 can be found at the web reference below.