Hallo aesoft. I love the replay viewer, and I tested the public beta of Pokemon Showdown.
For the replay viewer, there are some bugs.
You'll want to look into turn 55 of this replay. Specifically, Ferroseed's HP bar is displayed as 0%, after the opposing Azelf faints from Leech Seed damage, and it stays until Ferroseed really reaches 0%.
In this DPPt replay, Porygon-Z's damage is shown as 0% after sponging another Hydro Pump from Rotom-W, even though it survived (turn 25). Same thing happens next turn (turn 26) when Nidoqueen uses Earth Power on Lucario, it does 79% but then displays Lucario's HP bar as 0%.
I don't have much space to put all of the bugs here, so I'll redirect you to my post in the replay viewer thread.