"Each HTML element can belong to multiple content models" what does mean? 🤔

Answer 1

it means that elements may be block, inline or inline-block level. using css display for it

Answer 2

There are 7 content models: flow, metadata, phrasing, sectioning, embedded, heading, interactive.
Most of elements belong to flow content model and some other model.
For example, <audio> and <video> elements belong to flow, phrasing, embedded content models.
So, HTML element can belong to several models simultaneously.
About content models:
http://www.w3.org/TR/2011/WD-html5-20110525/content-models.html