We'll admit, we have a thing for nostalgia at Glamour—but when it comes to '90s fashion nostalgia, we go absolutely wild. So, on the occasion of Reese Witherspoon's 40th birthday (Happy birthday, Reese!), we took the liberty of going back to her style in the decade we came to know her in. Yes, dear readers, it's Reese Witherspoon's journey through our very favorite fashion decade, with cameos from Selma Blair, Ryan Phillippe, and yes, even a bare midriff or two. Here's Reese at 15 in 1991…