内部CSS弄乱了我的HTML文件……为什么?

I asked a question yesterday about how to have multiple HTML pages, and got a few answers saying I had to put them all in a folder. It worked great, thanks! : )
But another problem came up, and it doesn't make any sense... I tried to use inline styling, but that messed up the element. Confused, I switched to internal styling in the <head> element, and things worked fine.
When I added the second page to the site in index.html, it worked fine, until I added CSS to the second page, exactly like the first. The CSS worked, and the link disappeared from index.html.
I don't understand this at all, the only difference was that the second page was styled, and that effected the first page. Can someone explain Thanks!