Add a footnote regarding HTMLAudioElement

[ffxsam]

http://caniuse.com/#search=audio

Can we add a footnote that applies to Chrome (all versions) and IE11 and older? There's an issue when setting an audio element's currentTime to seek to a position and begin playing. If the media source is a VBR (variable bitrate) MP3, the position becomes out of sync with the actual audio.

This problem doesn't occur in Firefox or Safari.