JSFiddle - React, Tailwind, and code Playground

HTML

<link rel="stylesheet" href="http://ajax.googleapis.com/ajax/libs/jqueryui/1.8.4/themes/ui-lightness/jquery-ui.css">
<ul class="clearfix" id="topnav">
  <li class="mainNavFirst"><a href="/">  Our Home</a></li> <!-- bold this 'our' -->
  <li class="mainNavMiddle"><a href="#">Lorem Ipsum</a><!-- bold this 'Lorem' -->
    <div>
      <ul>
        <li class="sectionTitle">
          <h2><a href="#">Ipsum Lorem</a></h2>
        </li>
        <li><a href="#">Just a random link</a></li>
        <li><a href="#">Just a random link</a></li>
      </ul>
      <ul>
        <li class="sectionTitle">
          <h2><a href="#">Ipsum Lorem</a></h2>
        </li>
      </ul>
    </div>
  </li>
  <li class="mainNavLast"><a href="#">Another Link</a></li><!-- bold this 'Another' -->
</ul>

CSS

#topnav span {
    font-weight:bold;
}

JavaScript

$('#topnav > li > a').html(function(i,html){
    return html.replace(/^\s*([^\s]+)(\s|$)/, '<span>$1 </span>');
});