@Joshuan, that is in HTML. In HTML5 it is valid to use no quotes, but I recommend to use them. Because of the space problem.
@Cybershot, in CSS there is no need to use quotes. But it has the same problem as in HTML, if there is a space you need to use de quote. Examples:
font-family: 'Trebuchet MS', Verdana, sans-serif;
/* Trebuchet MS needs quotes, because there is a space in the name */
background: url('images/my image.png');
/* There is a space => quotes (but in this case, I should remove the space in the name */
/* No spaces, so we don't need to use quotes */