Two classes are written without spaces (two-class definitions)

The standard is to support a two-class definition, just as li.classname defines an attribute of Li with classname, and a space in the middle is defined as the element that has classname in the Li child element.

Class selector and write no space, like Li.classname, which can extend and write, ID and write (because the ID is unique, and write basically no meaning); ID and class and write should be able to form a control element style selector.

Example:

. cb-disable.selected corresponds to <div class= "cb-disable selected" ></div>

If there are spaces. Cb-disable. Selected corresponds to <div class= "cb-disable" ><div class= "selected" ></div></div>

Two classes are written without spaces (two-class definitions)