First test case:
1) Open/create CSS file
2) Add next content:

.myClass1, .myClass2 {
background-color: red;
}

3) Save changes
4) Choose any componenet on the page that support styleClass attribute.
5) Click button opposite styleClass attribute in Properties view
ASSERT: CSS Style Class Dialog should appear
6) Choose changed CSS file
7) Check Style class and choose line with classes from Step 2
8) Press Ok
RESULT: new styleClass attribute appears with next content: myClass1,.myClass2
POSSIBLE SOLUTION: style classes should follow one by one, throw white spaces

Second test case:
1) Choose any component with styleClass attribute
2) Set up the value of styleClass attribute in source code
3) Click button opposite styleClass attribute in Properties view
ASSERT: CSS Style Class Dialog appears but we haven't any preload into Style class field
4) POSSIBLE SOLUTION: the value of styleClass attribute should be displayed in the Style class field with a dot before