Just because you're a girl a skirt doesn't make a tank top garb
it's just never garb
Meh this one I don't really see a point of bringing up and really arguing. I can go and fight without a shirt on, or just in general be at an event. A female cannot. If a girl wants to wear a solid color tank top I'm not going to stress over it. Is it garb in a traditional sense? No, but does it fit a niche? Yes. There is always the argument of well you can always get _____ instead, but if that really doesn't seem to hold water given that it is in the extremes of temp. There can be considerations made can there not?
Why can't a girl wear a lightweight linen tunic instead of a modern tanktop? Or a sleeveless Roman style linen tunic (no more constricting than a tanktop, but looks much better and only takes a yard of fabric).
Or if she wants to show more skin, a leather bikini top or belly dancer wraparound halter top thingy that I don't know the name of [Ilsa says, 'cholli' - thanks!] (neither is really medieval, but they're solidly established staples in fantasy genres so they're good garb)?
Or a sleeveless dress, or an apron dress without an underdress (you show exactly
as much skin as a tanktop, but don't break any rules).
For that matter, you don't have to show skin at all to stay cool - desert tribes are smart, they cover up with fabrics that breathe to keep the sun off. I wear long sleeved wool in the summer partly because it's just as comfortable or more than short sleeves.
And that's not an exhaustive list!
Given that there are many historical and mainstream fantasy ways to stay cool in the heat that work just as well or even better than a tanktop without breaking the rules, why should we feel obligated to allow women to cheat the rules and wear tank tops? I really appreciate the concern that lies behind a desire to cut the girls a break, and I don't want to accidentally discriminate against women by requiring them to conform to a higher standard than the guys, but I don't think that the rules as written do put an unfair burden on women to wear hot, restricting garments while men get to go topless. There are lots of other options that let women stay comfortable without having to set a double-standard.