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In addition to the engine improvements, Opera 10 apparently scores 100 out of 100 on the Acid3 testing website. The Web Standards Project created the Acid tests to check a range of linking and rendering abilities in browsers, to encourage a standard baseline for coding. Theoretically, sticking to the test rules should ensure that websites can be seen properly on any browser, while reducing development costs.
For comparison, Firefox 3.0.4 scores 71 out of 100, while Firefox 3.1 Beta 1 scores 89 out of 100. Google Chrome 0.4 earns 79 out of 100, while Internet Explorer 7 struggles at 14 out of 100.