CSS in different browsers

So i have a stylesheet and it’s used in my header.php file. In firefox everything works fine, but in IE the styles don’t show up and i can’t figure out why.

For example I use <span class=”bigfont”>text here</span> then in my style.css i have:

.bigfont {
color: #000000;
text-decoration: none;
font-size: 16px;
font-weight: bold;
}

In firefox that works, in IE nothing, the text is just normal. What am i doing wrong?