In Page Content
html blocks in the page content are centered in a section or hero, so thats cool for stuff like videos and whatnot.
In Page Structure
HTML can also be added directly under the body, generally don't use this unless you know you're doing something bad. for instance the kinetic page is implemented completely inside an HTML block in the page body. this allowed kinetic to have a totally different design than what was supported by the CMS at the time.