Client side validation for rich text field in SharePoint 2010 using javascipt innertext property.
<script type="text/javascript">
var richtextboxid = document.getElementById('Your control id up to inplacerte');
if(richtextboxid.innerText.replace(/^\s+|\s+$/g,'')=="" || richtextboxid .innerTextinnerText.replace(/^\s+|\s+$/g,'')=="")
// Sometimes empty rich text box contains a square box so check for that box also
// Copy above code in notepad you will find that sqare box or you can find form IE developer tool
{
alert("Please enter Rich text value");
return false;
}
</script>
