JSFiddle - React, Tailwind, and code Playground

HTML

<table width="100%" leftmargin="0" topmargin="0" marginwidth="0" marginheight="0" border="0" cellpadding="0" cellspacing="0" bgcolor="#dedede" style="-webkit-text-size-adjust:none; line-height: 66%;">
	 <tbody>
		 <tr>
			 <td valign="middle"> <!-- The problem is valign work only with a height element for that cell -->
				 <!-- This next table is the one I want to centre --><table width="400" bgcolor="#ffffff" border="0" cellpadding="0" cellspacing="0" align="center" style="background:...

CSS

table { 
    width:400px;
    height: 300px;
    position:fixed;
    margin-left:-200px; /* half of width */
    margin-top:-150px;   /*half of height */
    top:50%;
    left:50%;
}