CODE EXAMPLE FOR HTML

consider adding a lang attribute to the html start tag to declare the language of this document.

Well, it depends on what your code is for. If you're coding a real website, you should
include the lang attribute because it's good for SEO. If you're making it for personal
use, you really don't need to.

TL;DR: If it's goin' on the web, use it. Otherwise, you don't have to.