The Opera browser is hugely popular on mobile devices, with more than 200 million monthly users.
"300 million marks the first lap, but the race goes on," said Lars Boilesen, CEO of Opera Software. "On the final stretch up to 300 million users, we have experienced the fastest acceleration in growth we have ever seen. Now, we are shifting into the next gear to claim a bigger piece of the pie in the smartphone market."
Described in a company blog post as "an under the hood" change, users will initially notice better site compatibility after the move to WebKit, especially with mobile sites; a new Opera mobile app built with WebKit will be demonstrated at Mobile World Congress at the end of February.
WebKit is layout engine software designed to allow web browsers to render web pages, and is used to power the Apple Safari and Google Chrome browsers, giving it the most market share of any layout engine, at over 40 percent.
As for mobile use, WebKit is used in the experimental browser for the Amazon Kindle, as well as the default browser for iOS, Android, BlackBerry, Bada, Tizen and webOS.
The Norwegian firm's web browser is used mostly by mobile devices, with the Opera Mobile and Opera Mini apps clocking up 229 million monthly users between them in December.
Looking forward to the partnership with WebKit, which is also based in Oslo, Opera employee Bruce Lawson said it "now has the kind of standards support that we could only dream of when our work began. Instead of tying up resources duplicating what's already implemented in WebKit, we can focus on innovation to make a better browser."
Lawson added: "In our internal builds, we've experimented with adding support for some new standards and enhanced some features that were lacking compared with Presto (for example, multi-column layout)."
CEO Boilesen concluded his statement: "300 million is a bonanza big number, and we are certain this move will help us grow even more - and make our products even better. Opera is for people who appreciate choice, and we are going to make it even easier to choose Opera in the future."
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