<apex:page standardController="Opportunity">

    <style>
        .ReqHelpText {
            display : inline-block;
            width : 3px;
            background-color : #c00;
        }
    </style>

    <apex:form>
        <apex:pageblock >
            <apex:pageBlockSection collapsible="false" showHeader="true" title="Opportunity Section">
                <apex:facet name="header">
                    <apex:outputPanel>
                        <apex:outputPanel styleClass="ReqHelpText">&nbsp;</apex:outputPanel>
                        <apex:outputText value="Fill Required Fields"/>
                    </apex:outputPanel>
                </apex:facet>

                <apex:inputField value="{!Opportunity.Name}"/>
            </apex:pageBlockSection>
        </apex:pageblock>
    </apex:form>
</apex:page>