We announced that Opera will switch its rendering engine to WebKit and JavaScript engine to V8 by building on top of Chromium (the open-source browser that powers Google Chrome).
The short answer is that it shouldn’t affect your day-to-day work. Keep coding to the standards, not to individual rendering engines; test across browsers - Opera, Firefox, Chrome, Safari and > Internet Explorer; use all vendor prefixes and an unprefixed form in your CSS and JavaScript. However, it remains important to keep the following in mind:
Chromium, and therefore future versions of Opera, has built-in support for the WebM, Ogg Theora and Ogg Vorbis media codecs but does not natively support H.264 or MP3 media codecs (although if these are installed in a device’s operating system it will allow that to render media). The correct way to check support is with HTML5 canPlayType. The simplest method to ensure all modern browsers receive the correct codecs is to encode in both WebM and H.264 and provide two elements or use canPlayType to check support (see Introduction to HTML5 video for more information).
The window.opera object will not exist in future versions of Opera. We continue to recommend that developers SHOULD NOT use browser-sniffing; feature-detection - either using a 3rd party solution such as Modernizr or hand-rolling your own - is better.
I use Modernizr extensively in my last few projects. Highly recommended.