In the first look review of the new Opera Mini browser, AppleInsider noted that the browser did a poor job of rendering basic layouts of text and tables on the NYTimes.com website (as shown below).
Opera Mini's weak rendering leaves many page elements unreadable. This is a rather stark contrast to Opera's desktop browser, which has consistently maintained high web standards compliance competitive with Apple's Safari browser. Each uses a different rendering engine: Safari uses WebKit, while Opera uses its own Presto.
Opera Mini scales back rendering quality by stripping a variety of features from web pages as they pass through Opera's proxy server. Fonts appear to all be replaced with Helvetica, graphics can be heavily compressed, and the entire structure of the page is translated from HTML into a markup language called OBML (Opera Binary Markup Language).
The Acid Test
Browsers can score 100/100 and still fail the test if they do not correctly render the final page of graphics to a pixel-level precision. Additionally, there is also a performance component to the text, although this is not usually taken into consideration on mobile browsers.
When benchmarked by the site iSmashPhone, the new Opera Mini failed with a score of 74/100, while Safari Mobile received a score of 100/100.
iPhone's Safari browser tough to beat in Acid3 standards compliance
The site didn't note that the current iPhone OS Mobile Safari, despite its score, does not actually pass the Acid3 test due to rendering details (noted by arrows, below). However, it does do much better on the test than any other mobile browser (even browsers that use the same WebKit rendering engine that Safari uses), and better than many other desktop browsers as well.