Explanation
The p DOM Element can only contain inline elements. (MDN) (W3C HTML5)
The start tag is mandatory. The end tag may be omitted if the <p> element is immediately followed by an <address>, <article>, <aside>, <blockquote>, <div>, <dl>, <fieldset>, <footer>, <form>, <h1>, <h2>, <h3>, <h4>, <h5>, <h6>, <header>, <hr>, <menu>, <nav>, <ol>, <pre>, <section>, <table>, <ul> or another <p> element, or if there is no more content in the parent element and the parent element is not an <a> element.
Permitted tags
Here is an exhaustive list of the permitted tags inside a p element.
Content:
<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 consisting of white spaces characters).