VP8 has recently been offered by Google as an open video compression format in attempt to compete with the widely used H.264 video compression standard. This paper describes the major differences between VP8 and H.264 and provides detailed comparative evaluations through extensive experiments. We use 29 raw video sequences, offering a wide spectrum of resolutions and content characteristics, with the resolution ranging from 176x144 (QCIF) to 3840x2160 (2160p). To ensure a fair study, we use 3 coding presets in H.264, each with three types of tuning, and 7 presets in VP8. The presets cover a variety of achieved quality or complexity levels. The performance metrics include accuracy of bit rate handling, encoding speed, decoding speed, and perceptual video quality.