Why can't the tag contain a tag inside it?

Question

As far as I know, this is right:

  
some words

But this is wrong:

  &l         


        

Answer 1

According to HTML5, the content model of div elements is flow content

Most elements that are used in the body of documents and applications are categorized as flow content.

That includes p elements, which can only be used where flow content is expected.

Therefore, div elements can contain p elements.

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However, the content model of p elements is Phrasing content

Phrasing content is the text of the document, as well as elements that mark up that text at the intra-paragraph level. Runs of phrasing content form paragraphs.

That doesn't include div elements, which can only be used where flow content is expected.

Therefore, p elements can't contain div elements.

Since the end tag of p elements can be omitted when the p element is immediately followed by a div element (among others), the following

  
some words

is parsed as

some words

and the last

is an error.