I'm learning CSS "seriously" for the first time, but I found the way you deal with multiple CSS classes in CSS and HTML quite incoherent.

For example I learned that if I want to declare multiple CSS classes with a common style applied to them, I have to write:

.style1, .style2, .style3 {
   color: red;
}

Then, if I have to declare an HTML tag that has multiple classes applied to it, I have to write:

<div class="style1 style2 style3"></div>

And I'm asking why? From my personal point of view it would be more coherent if both could be declared by using a comma to separate each class, or if both could be declared using a space; after all IMHO we're still talking about multiple classes, in both CSS and HTML. I think that it would make more sense if I could write this to declare a div with multiple classes applied:

<div class="style1, style2, style3"></div>

Am I'm missing something important? Could you explain me if there's a valid reason behind these two different syntaxes?