JSFiddle - React, Tailwind, and code Playground

HTML

<input type="hidden" name="valida" id="valida" value="ok" />
<table border="0" bgcolor="#E9EAED" style="margin:0px 0px 30px 0px;" width="100%" cellpadding="0" cellspacing="0">
    <!-- <?php if($_SESSION[ 'FBID'] || $_SESSION[ 'user_id']){ ?> -->
    <tr>
        <td valign="top">
            <div id="flash" align="left"></div>
            <div id="show" align="left"></div>
        </td>
    </tr>
    <tr>
        <td valign="top" width="7%">
            <div style="padding:15px 5px 5px 20px;">
                <img width="33" height="33" src="<?php echo $_SESSION['user_foto'] ?>" />
            </div>
        </td>
        <td valign="top" width="93%">
            <div style="padding:15px 20px 15px 5px;">
                <input type="text" style="width:100%; height:33px;" placeholder="Escreve um comentário..." id="comentario" name="comentario" value="" />
            </div>
            <input type="button" id="submit" style="display:none;" class="submit_button" />
        </td>
    </tr>
</table>

CSS

#flash {
    display: none;
}

JavaScript

function enviar() {
    var input = document.getElementById('comentario');
    var textcontent = input.value;
    var data = {
        id_estabelecimento: "<?php echo $row -> id; ?>",
        user_id: "<?php echo $_SESSION['user_id']; ?>",
        opiniao: textcontent
    };

    $("#flash").html('<span class="load">Aguarde por favor..</span>').fadeIn(800);
    $.ajax({
        type: "POST",
        url: '/echo/html/',
        data: {
            'html': input.value
        },
        cache: true,
        success: function (html) {
            $("#show").append('<div>' + html + '</div>');
            input.value = '';
            $("#flash").hide();
            input.focus();
        }
    });
}
$(function () {
    $('#comentario').on('keydown', function (e) {
        if (e.which != 13) return true;
        enviar();
    });
    $('form').on('submit', function (e) {
        e.preventDefault();
    });
});