html can go anywhere between the <body></body> tags, where you want the code to run.
A modern webpage is in 2 parts, the content and the layout.
The content is html or a script/program creating dynamic html content.
and the layout is in CSS (Cascading Style Sheet).
I'm guessing that the website using the term Layout means the body of the page. Unless it's not html code, maybe css or java, or php ...