without pure css this is impossible (as you don't want a separation in the markup)...
<span>Monty Burns</span><br />
<span>Bart Simpson</span>
<script type="text/javascript">
$(document).ready(function() {
var spans = $('span');
spans.each(function(index, element) {
var span = $(element);
var spanText = span.text();
var spanTextArray = spanText.split(' ');
var spanTextArrayLength = spanTextArray.length;
var lastName = spanTextArray[spanTextArrayLength -1];
spanTextArray.pop();
var firstName = spanTextArray.join(' ');
span.text(firstName);
var spanLastName = $('<span/>');
spanLastName.css('font-weight', 'bold');
spanLastName.css('margin-left', '5px');
spanLastName.appendTo(span);
spanLastName.text(lastName);
});
});
</script>
working demo.
edit: if you do not want an extra span
-tag in there, just change
var spanLastName = $('<span/>');
spanLastName.css('font-weight', 'bold');
to
var spanLastName = $('<strong/>');