They are only as visible as the player is. if the player hides behind a wall, the tags will too.
note: it will not work on pre-0.4.14 servers, and if client join with pre-0.4.14 the normal tags will be visible to them.
Some code taken from gauges (CC0 1.0)
And also some code and textures from npcf (LGPL for code, WTFPL for textures)
My part of the code is WTFPL.
Github: download latest browse code