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

It is not valid. #width: 75%; is a syntax error, since # isn't used in CSS property names (although it is used in CSS selectors, to select elements with specific ids). Most browsers will ignore it (hopefully) and only the first rule will be applied.

It might have been someone's attempt to write a CSS comment. This is the valid way: /*This is a comment*/

Edit

I would suggest using a CSS reset file to account for browser differences.