There is a top yellow box in this web that its annoying and i try to hide it using the xpath but it dindt succes cause the entire page it was erased. Later i realised that i have to modify the code with the selector first-child and it worked.
Basically the operator > it's useful for the parent structure but if there is many elements in cascade and u want to apply css code to the first element only, u have to include with first-child. If u want the nth element to be applied, i think that the selector is: nth-child(x) or something like that, I really haven't used yet.

Note: Thanks Valacar for the code, really it works the same as mine, the purpose it was to hide the yellow box at top, i dont mind the line around. Really yr code is better because i dont hide all the table but the td element (that is inside the table). I am not a programmer and i am beggining with CSS language so excuses if i am wrong.