> Moreover, besides self-closing tags, in HTML5 there are many optional tags. Those are (rooted in) SGML elements having start-element tag omission allowed, wh

> Moreover, besides self-closing tags, in HTML5 there are many optional tags. Those are (rooted in) SGML elements having start-element tag omission allowed, whereas HTML's "self-closing tags" (in HTML5 parlance) correspond to elements having end-element tag omission allowed. In your example, SGML infers the missing tags for html, head, body, and p to arrive at the canonical form: <html> <head> <title>Example Webpage</title> </head> <body> <h1>Welcome to my webpage</h1> <p>This is some content!</p> </body> </html> Cf. http://sgmljs.net/blog/blog1701.html http://sgmljs.net/blog/blog1701.html (the TALK slides) explaining the inferences SGML does on the example input in detail