Accessory is a tag that could use a lot more fleshing out. There's plenty of tags that should probably imply it, but it raises the question of what classifies as an accessory.
Wikipedia mentions these as accessories (not a complete list, just ones I think are worth mentioning):
List
Most headwear and other clothing items are also listed, but I think since e621 considers that clothing, it definitely shouldn't fall under accessory.
https://en.wikipedia.org/wiki/Fashion_accessory
There's definitely way more tags floating around that should probably fall under accessory. I think it'd be great if we could set up accessory as an umbrella tag similarly to clothing, but that first requires us to figure out what we want to fall into it.
Updated