I will take the link pointed by nir :
Wrap the fieldset in a div with the display: table; margin: 0 auto; style display: table; margin: 0 auto; display: table; margin: 0 auto; . This may be a bit hacky, but it works without having to set the width and margin.
<div style="display: table; margin: 0 auto;"> <fieldset data-role="controlgroup" data-type="horizontal" data-theme="b" style="font-size:12px;border:2px;"> <input type="radio" data-theme="b" name="radio-choice-b" id="radio-choice-wuser" value="windowUser" checked="checked" /> <label for="radio-choice-wuser" style="font-size: 12px;" class="ui-btn-section-active" id="lblWindowUser">win user</label> <input type="radio" data-theme="b" name="radio-choice-b" id="radio-choice-muser" value="mfileUser" /> <label for="radio-choice-muser" style="font-size: 12px;" id="lblMfileUser">M file user</label> </fieldset> </div>
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