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Mar 05, 2018

It is safe to assume that all HTML5 web browsers will honor HTML 4 websites, but only if the appropriate HTML 4 document type declaration is found.

This maintains backward compatibility with the vast majority of websites on the Internet. The HTML 4 declaration instructs the web browser to run in HTML 4 standards mode, thus rendering the older standard correctly.

An example of an HTML 4 document type declaration:

<!doctype html public “-//W3C//DTD HTML 4.01//EN” “http://www.w3.org/TR/html4/strict.dtd”>