What are the allowed elements within the P tag?

The <p> element is categorized as flow element and < a href="https://html.spec.whatwg.org/multipage/dom.html#palpable-content-2"> palpable element , and can therefore be used anywhere an element is expected flow. Allows as child elements any element that is a phrase element .

The definitions of each other, in addition to the links mentioned, are found in W3C, 3.2 Elements , 3.2.5 Content models , 3.2.5.1 Kinds of contents :

• 3.2.5.1.1 Metadata content
• 3.2.5.1.2 Flow content
• 3.2.5.1.3 Sectioning content
• 3.2.5.1.4 Heading content
• 3.2.5.1.5 Phrasing content
• 3.2.5.1.6 Embedded content
• 3.2.5.1.7 Interactive content

The relationship between the groups is:

Source and version of the iterative image: link

In a simplified way, sentence elements are the elements that define the content of the application or its formatting - it is worth mentioning the word formatting here, since formatting is not stylized (CSS applies styles).

At the moment, the phrase elements are (links to the documentation you find here ):

  

a, abbr, area (if descending from a <map> ), audio, b, bdi, bdo,   embed, dfn, datalist, datalist, dite, date,   i, iframe, img, input, ins, kbd, label, link (if allowed in   body), map, mark, MathML math, meta (if you have the attribute    itemprop ), meter, noscript, object, output, picture, progress, q,   s, script, select, slot, small, span, strong, sub, sup,   svg, template, textarea, time, u, var, video, wbr, autonomous custom   elements and text .

If other elements are added as children of <p> , it is possible for the element to close earlier than expected when the client processes the HTML structure, as Rafael commented on your answer .

The specification also explicitly cites the example of defining lists within a paragraph, where it is not possible to use elements <ol> and <ul> within <p> .

p > ul {
  color: red;
}

<p>
  <ul>
    <li>Item 1</li>
    <li>Item 2</li>
  </ul>
</p>

The interpreted HTML will be:

<p></p>
<ul>
  <li>Item 1</li>
  <li>Item 2</li>
</ul>
<p></p>

So much so that CSS, which defines the red font for a ul element within a p , did not work. In this case, the ideal is, as it was interpreted, to generate two distinct paragraphs, one before and another after the list or define the content within another element, as well as div .

<div>For instance, this fantastic sentence has bullets relating to
<ul>
 <li>wizards,
 <li>faster-than-light travel, and
 <li>telepathy,
</ul>
and is further discussed below.</div>

That will be interpreted exactly the way it is.

The <p> tag defines a paragraph.

Within the <p> tag, any plain text content and markup is allowed, such as: <abbr>, <audio>, <b>, <bdo>, <br>, <button>, <canvas>, <cite>, <code>, <command>, <data>, <datalist>, <dfn>, <em>, <embed>, <i>, <iframe>, <img>, <input>, <kbd>, <keygen>, <label>, <mark>, <math>, <meter>, <noscript>, <object>, <output>, <progress>, <q>, <ruby>, <samp>, <script>, <select>, <small>, <span>, <strong>, <sub>, <sup>, <svg>, <textarea>, <time>, <var>, <video>, <wbr>

If you insert within the <p> tag some element such as: <address>, <article>, <aside>, <blockquote>, <div>, <dl>, <fieldset>, <footer>, <form>, <h1>, <h2>, <h3>, <h4>, <h5>, <h6>, <header>, <hr>, <menu>, <nav>, <ol>, <pre>, <section>, <table>, <ul>

The paragraph will be automatically closed. Let's look at the example:

<p> teste <div> div </div> </p>

By analyzing the source code, the <p> tag will close automatically.

<p> teste </p>
 <div> div </div>
<p></p>

Sources : The Paragraph element , Content with Allowed Text , W3C .

It's about web semantics.

The "p" comes from a paragraph, so it is expected that the content of this tag will have plain text and, at most, text formatting tags, although this can be done more appropriately with CSS.