I just tried it and it seems to work. If it is not standard, I need to use many divs, all of which are the same except for the background color. How should I make them conform to the standard?
The same properties seem to be overridden according to the order in the css file.
Refer to w3cschool
http://w3schools.com/html/default.asp
This does not violate
Do not use non-standard sticky notes and css attributes in the page
It is compliant, just separate multiple attributes with spaces
.a {}
.b {}
div class="a b"
That’s it, no problem.