How to make only placeholder italics in input

Sure. Just apply the style to these CSS rules:

::-webkit-input-placeholder {
   font-style: italic;
}
:-moz-placeholder {
   font-style: italic;  
}
::-moz-placeholder {
   font-style: italic;  
}
:-ms-input-placeholder {  
   font-style: italic; 
}

Probably not all of these rules are needed. I always just reference CSS Tricks because I always forget how to do it.

Edit: Note that these rules cannot be combined into one selector. They must be declared separately as noted by Dave Kennedy in the comments below.