Guild Wars 2

I was reading some post on reddit when i found this:
http://www.reddit.com/r/Guildwars2/comments/36ca19/be_reminded_ingame_when_a_boss_is_about_to_start/

OK !? Its just beautifull and my eyes lit up when I saw it for the first time. BUT after read the coments i start to think about it and came here to ask. Is it legal for GW2? Is it a valid APP?

here is the video:
https://www.youtube.com/watch?v=r8VbXJHtZZA

This has nothing to do with the GW2 API.

And yes it is legal since it is just an overlay.