Why do we include at the top of all our HTML documents?
It informs the browser how the document should be interpreted, by indicating what version or standard of HTML is being used.
Without this doctype declaration, modern browsers may run in quirks mode. This is a mode used by modern browsers to interpret CSS. However, this interpretation adheres to non-standard behavior. The reason that browsers have this mode is to keep backward compatibility for websites that were built before web standards were put in place.
Having the doctype declaration at the top of HTML documents cause the browser to activate full Standard mode.