WebM defines the file container structure, video and audio formats. WebM files consist of video streams compressed with the VP8 or VP9 video codecs and audio streams compressed with the Vorbis or Opus audio codecs. An encoding speed versus efficiency comparison of the reference implementation in libvpx, x264 and x265, libvpx was mostly superior to x264 across the range of comparable encoding speeds. In contrast to HEVC, VP9 support is common among web browsers. The combination of VP9 video and Opus audio in the WebM container, as served by YouTube, is supported by roughly 75% of the browser market as of early 2017. Android has supported VP9 since version 4.4 KitKat, though hardware acceleration varies.