class overrule when two classes assigned to one div

Answer 1

Many classes can be assigned to an element, you just separate them with a space

Because of the cascade in CSS, the overwriting rules closest the to bottom of the document will be applied to the element.

So if you have

.myClass
{
    background: white;
    color: blue;
}

.keepOnClassing
{
    color: red;
}

The red color will be used, but not the background color as it was not overwritten.

You must also take into account CSS specificity, if you have a more specific selector, this one will be used:

.myClass
{
    background: white;
    color: blue;
}

div.myClass.keepOnClassing
{
    background: purple;
    color: red;
}

.stayClassySanDiego
{
    background: black;
}

The second selector here will be used as it is more specific.

You can take a look at it all here.