How to level audio for the internet has come up several times. The safe answer is to say there is no standard.
Apple addressed this issue as early as 2001 with its SoundCheck technology. The backbone of iTunes levels norming, it is based on an earlier version of BS1770, predating the EU and US broadcast regulations by several years.
Most of what gets uploaded is too loud. The broadcast audio levels sound really wimpy on YouTube. It happens that the Apple feature normalizes to about -16 LUFS or -17 LKFS, depending which side of the pond we're on.
That's about 6dB louder than broadcast. So maybe there is a de facto standard, considering iTunes' music market share.
Just wanted to start some conversation on a subject that needs a durable solution.