Remove quotation marks of sans-serif, and put it at the end of declaration

Sans-serif shouldn’t be quoted like "sans-serif", or modern browsers do not recognize. Leave it unquoted.
Also, sans-serif should be put at the end of the font-family CSS property for it is a fallback and nobody knows what font it exactly is, and a browser may stop recognizing following font names because for almost every code point there is a sans-serif it actually falls back to.
Probably, we wouldn't add this comment to the official source. :smile: