Do you think that this approach is correct? Opera has less than 3% in the market. Should I develop my project to support Opera? I intend no to. I want to do this using HTML5 and as far as I know not all elements of new version of html are supported by Opera. What is your approach?
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You should develop with interoperability in mind as to develop your Web sites in a way that will work across devices and softwares.
Browser Market Shares
The global market share stats are hiding a lot of juicy details and you would be surprised by the data if you look at it closely. Statcounter is a way to get an idea of stats. Just be careful, because these are just collecting data with counters. But you could for example compared the world market shares for Desktop with the Russian Federation market shares.
Or you could check the world market shares for Mobile with the ones of let say Mexico and USA and you would notice that two neighboring countries have very different market shares.
Web Specifications supported by Opera
To get an understanding of what is supported or not supported yet by Opera, I would recommend you read the Web specifications support in Opera products. It gives a detailed list of the areas of support not only for Opera desktop but all type of Opera browsers (Mobile and Mini included).
If you check the user agent string, you can also get the version of the rendering engine, for example, Presto 2.10, and then head to the specific Web technologies support page for this engine. It would help to assess what are Web technologies supported.
Another good site that will help you assess the support of Web features across the browsers is When Can I use. For example for CSS Animation and CSS transitions.