jQuery Official Doc.

<script src="http://www.google.com/jsapi"></script>
<script>
    // Load jQuery
    google.load("jquery", "1.3.1");
</script>
<script>
    $(function() {
        $("#my_form_id input").blur(function(e) {
            // stop normal blur
            e.preventDefault();
            var my_input = $(this);
            // send request
            $.ajax({
                type: "POST",
                url: "wwv_flow.show",
                data: {'p_arg_names':["G_VALIDATE_ITEM", "G_VALIDATE_ITEM_VALUE"],
                       'p_arg_values':[$(this).attr('id'), $(this).attr('value')],
                       p_flow_id:$('#pFlowId').attr("value"),
                       p_flow_step_id:$('#pFlowStepId').attr("value"),
                       p_instance:$('#pInstance').attr("value"),
                       p_request:"APPLICATION_PROCESS=ITEM_VALIDATION_001"
                },
                dataType: "json",
                success: function(data, textStatus){
                    $("#"+data[0].input_id+"_error").remove();
                        
                    if (data[0].error == "REQUIRED") {
                        $("label[for='"+data[0].input_id+"']").after("<div id='"+data[0].input_id+"_error' class='t4InlineError'>"+data[0].message+"</div>");
                        my_input.focus;
                    }
                }
            });
        });
    });  
</script>

DECLARE
/*
*This process validate mandatory fields.
 */
    l_retval            VARCHAR2(32767);
    l_required_flag     NUMBER := 0;
    l_error             VARCHAR2(255);
    l_message           VARCHAR2(255);
BEGIN

    SELECT  1
    INTO    l_required_flag
    FROM    apex_application_page_items
    WHERE   application_id = wwv_flow.g_flow_id
      AND   page_id = wwv_flow.g_flow_step_id
      AND   upper(item_name) = upper(:G_VALIDATE_ITEM)
      AND   instr(lower(item_label_template),'required') > 0;
      
    IF l_required_flag = 1 AND trim(:G_VALIDATE_ITEM_VALUE) is null THEN
        l_error := 'REQUIRED';
        l_message := 'Please enter a value.';
    END IF;
    
    l_retval := '[{''input_id'':'''||:G_VALIDATE_ITEM||''',''error'':'''||l_error||''',''message'':'''||l_message||'''}]';
    
    htp.prn(l_retval);
EXCEPTION
    WHEN no_data_found THEN
        htp.prn('[{''input_id'':'''||:G_VALIDATE_ITEM||''',''error'':''NO_DATA_FOUND'',''message'':'''||SQLERRM||'''}]');
    WHEN others THEN
        htp.prn('[{''input_id'':'''||:G_VALIDATE_ITEM||''',''error'':''OTHERS'',''message'':'''||SQLERRM||'''}]');
END;