Opera displays a number of errors in the built-in error console (available through the Opera menu, page> Developer Tools> Error Console).
You can also use the Opera Dragonfly debugger (similar to Firefox Firebug) to catch all kinds of errors in the content (css, javascript, svg, etc.). It is also available through the Opera menu, Page> Developer Tools> Opera Dragonfly, or by right-clicking on the contents and selecting "Inspect Element".
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