setAttribute and class conflict resolution_javascript skills

Suppose we want to use setAttribute to change a dom class, how should we write it?
1. setAttribute("class","red")
2. setAttribute("className","red")

The answer is:
Option 1. Use under firefox
Option 2. Use

under IE. If you setAttribute("class") to DOM under IE and use IE Developer to look at the structure, you will find that two classes are displayed on the DOM... It turns out that IE is the Custom classes are distinguished from system classes. - -b