Making clothes that are 97% biodegradable is surprisingly easy. Pick a natural fibre fabric, buy some 100% cotton or linen thread, and you're there. (You can even get 100% cotton overlocker thread - though you have to go searching for it.)
Those last few percent, though? That can be tough. One of the biggest offenders is closures. While there are plenty of clothes that don't need something to hold them shut, I confess I like having the option. Below is a round-up of various closures, and their compostability.
Note: for the purpose of this article, I'm treating metal closures as "compostable", in the sense that they're 1) easy to remove before you compost the rest of the object, and 2) infinitely reusable on other things, unless they break - at which point they're usually recyclable using our current commercial systems. And remember, re-using things that already exist is always preferable to buying new, "better" items.