Anything goes; to blend in with some of the Liverpool lasses you could wear your skimpiest dress, lots of make up, maybe a huge hat & not a lot else.
If thats not really your style then a nice pair of trousers with boots or shoes that are comfy for lots of walking & a smart coat.
Don't need to be too formal for Tatts - you willl see some sights, liverpool ladies in their skimpy finery plastered in fake tan but don't feel you have to join in (unless you want to!)
Need sensible shoes - a bit of a hike from park & ride and you will probably be standing most of the time, and it can get cold. If you feel like dressing up a bit (and who doesn't for a day at the races) suggest a trouser suit and boots with a warm coat over the top.
Have a great day - my OH and best friend took me for my 30th and altho it poured we had a fab time!
Tattersalls is generally a bit rough and ready. Some people will dress up but a lot won't. I'd go comfortable particularly your shoes and take some waterproof gear in case the rain does turn up.