HTML class attribute with spaces, it is a W3C valid class?

Question

About HTML class attribute, that assigns CSS class (or classes?) to a tag. The use of spaces, like in

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Answer 1

That won't work in the CSS file OR the HTML.

means the div element has class a AND class b AND class c.

Meanwhile, on the stylesheet side of things, .a b c { property: value; } is not valid because it literally means "element c with ancestor b with ancestor having class a" (and b and c are obviously not valid HTML elements) while .a .b .c { property: value; } would mean "element having class c with ancestor element having class b with ancestor element having class a". Look up CSS specificity rules if this makes no sense to you.

Use dashes or underscores instead of spaces.