You can try the following in ZK 5 Sandbox, the CSS behind font-family (included) will not set to this label.

<label value='Test' style='color:#0000CC; font-family:"courier new","times new roman","黑体"; font-size:30px;'/>

The only way can make it work is exchange single quotation mark with double quotation mark:

<label value='Test' style="color:#0000CC; font-family:'courier new','times new roman','黑体'; font-size:30px;"/>

The first one works with ZK 3.6.x.

related forum thread:
http://www.zkoss.org/forum/listComment/18085-zk5-If-double-quotation-mark-exists-in-CSS-style-style-will-not-properly-set