Re the "10% rule" on design
Mick is correct in that there is no specific ADR thats specifies a trailer has to have <10%+ on the tow ball, however, the front axle, or front axle of any set of axles cannot be before the centre of the trailer, and the weight behind the front axle must be less than what is in front of the leading axle.
That is the interpretation (and I stress it is an interpretation), of the design rules from engineers who design and draft trailers for a living. Goty that from 2 of them and all they do is draft trailers from light domestic to trucking grade.
The biggest cause of trailer "sway" is where the load is behind the leading axle, and it seems that good engineering was working to limit that possibility, by building the trailer accordingly for starters.
That said, the rules vary between states, and in WA, the licencing police were quite adamant that they wanted to see 10%+ on the tow ball, or they would not licence my trailer.
I built with a tri axle and wanted to put front axle forward of centre with the two rear axles behind centre, but seems they were not happy with that either, so i ended up with maybe 8% on the tow ball (after I "adjust" the spare wheel location).
Bottom line is, smart drivers load their trailers being mindful of load distribution and towing limts of their vehicle, but a great many drivers appear totally oblivious to any such concerns, and that is likely what the police are starting to target.