Trying to change the placeholder's font-weight

I have an input box with a place holder and the placeholder text is very bold and I would like to make it thinner. I looked at this resource http://ift.tt/1NNunoa, which gave the code to change the font-weight but it didn't work. This is my code so far:

.infoPlaceholder::-ms-input-placeholder {
    font-weight: lighter;
}

<input class="infoPlaceholder" name="firstName" placeholder="First Name"/>

However, this did not change my placeholder's weight. It seems still pretty bold to me. Is there something wrong with my code? Or is there another way to produce this effect?