What is !DOCTYPE html in HTML5?
The Document Type Declaration needs to be present at the beginning of a document. The DOCTYPE is an instruction to the web browser about the version of markup language in which a web page is written. The DOCTYPE is not actually an element or HTML tag but it needs to be the first thing in your web page document. In HTML, the DOCTYPE is case insensitive always.
The three conforming variants of the HTML5 DOCTYPE declarations are as:
<!DOCTYPE html SYSTEM "about:legacy-compat" >
<!DOCTYPE html SYSTEM 'about:legacy-compat' >
The term "legacy-compat" or 'legacy-compat' refers to compatibility with legacy producers only.
The above quoted string "about:legacy-compat", which must be written in lower case. This quoted string, however, may also be quoted with single quotes, rather than double quotes.