Dual Selector Demo

class='tight' this text should have letter-spacing: -.05em;

class='heavy' this text should have font-weight: 900; and color: #900;

class='tight heavy' this text should have font-weight: 900; and letter-spacing: 0; and color: #009;

Mac IE Screenshot

Mac IE description:
class='tight'
this text should have letter-spacing: -.05em; (it does)

class='heavy'
this text should have letter-spacing: -.05em; (it does not, it's color is #009 and
letter spacing is 0)

class='tight heavy'
this text should have font-weight: 900; and letter-spacing: 0;
and color: #009; (it does)