After all I have read and seen, it seems that the standards for LARP weapons are depending on the country/area where the LARP takes place.
The technical standards in Germany (at least, probably more countries in Europe), if selfmade, is pretty much:
a bar of fiberglass as center, to stabilize the weapon, then some layers of mats made of foam material. the fiberglass bar has to be shorter than the weapon itself, and (I think it's best practice) to make it even more safe, you put/glue some leather around the tips (of the bar, so you won't see it on the outside). After that just glueing the layer of mats of foam material, and when it's dry, coloring it and complete it with a layer of latex or topcoat.
In short: fiberglass bar as core, ends of the bar secured with leather, and some layers of foam material mats around it. Safety always comes first! (Though it still is also important that it also looks good/normal.
The styling standards are... well... depending on the group and the kind of LARP you are playing. A little bit of personal preference, but also thinking of a little bit realism. (i.e. don't make the weapons way too large. This looks nice in some Final Fantasy games, but pretty much ridiculous if the character holding it isn't some kind of very, veeery mighty and strong... thing.)