Install FFmpeg with homebrew. You'll need to install it with a couple flags for webm and the AAC audio codec.
brew install ffmpeg --with-libvpx --with-libvorbis --with-fdk-aac --with-opus| img { | |
| opacity: 0; | |
| -webkit-transition: opacity .2s; | |
| -moz-transition: opacity .2s; | |
| -o-transition: opacity .2s; | |
| transition: opacity .2s; | |
| } | |
| img:loaded { | |
| opacity: 1; |