For a number of years web standards used various forms of HTML 4.
The HTML 5 specification was finally completed in 2014 although many browsers already incorporated elements of the specification so there was no real delay before developers could start to use it. Now, the latest browsers largely support the specification.
HTML 5 takes over from all the previous specification and is the one that should be used from now on.
HTML 5 incorporates many new features particularly for multi-media and graphical content and adds a new page structure using “schematic mark-up elements” to make layouts easier to understand.