Is a CSS property starting with a hash (#) valid?

Question

What does the following CSS do and is it valid?

h4 {
    width: 83%;
    #width: 75%;
}

Answer 1

BTW if the second width was not preceded by #, it would take width = 75% and not 83%. The last value always overrides all the preceding ones. As others pointed out, it could be a hack, which I don't know but most likely a syntax error.