We read here:
MDN <p>
Permitted content Phrasing content .
The link says:
Elements belonging to this category are
<abbr>, <audio>, <b>, <bdo>, <br>, <button>, <canvas>, <cite>, <code>, <command>, <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>
and plain text (not only stars of white spaces).
If only this condition is fulfilled:
<a>, if it contains only phrasing content <area>, if it is a descendant of a <map> element <del>, if it contains only phrasing content <ins>, if it contains only phrasing content <link>, if the itemprop attribute is present <map>, if it contains only phrasing content <meta>, if the itemprop attribute is present
speaking Russian, inside <p> can be only elements from the first list, and elements from the second with restrictions:
<a><del><ins><link><map> - can contain only the elements of the first list, or recursively satisfying these three conditions<area> only if it is inside the map<meta> only if there is an itemprop attribute
I hope clearly explained.