Why text in apex:page BlockSection is changed when it is reloaded by ajax in Salesforce visualforce?

I have code following. When I click on “Click” button, text “????” in apex:pageBlockSection is changed into “ஷ”. Why the text changed? Thanks all !

<apex:page>
<apex:pageBlock id=”san7″>
<apex:pageBlockSection>
????
</apex:pageBlockSection>
</apex:pageBlock>
<apex:pageBlock>
<apex:outputPanel layout=”block”>
<apex:form>
<apex:commandButton value=”Click” reRender=”san7″/>
</apex:form>
</apex:outputPanel>
</apex:pageBlock>
</apex:page>