<div class="form_style">
    <fieldset>
      <label for="contentText" class="fixedwidth">Name:</label>
      <input type="text" name="content_txt" id="contentText" class="inputtext" ><br/>
    </fieldset>
    <fieldset>
      <label for="address" class="fixedwidth">Address:</label>
       <input type="text" id="address"class="inputtext" /><br/>
    </fieldset>
    <fieldset>
       <label for="lat" class="fixedwidth">Lat:</label>
     <input type="text" id="lat"class="inputtext" /><br/>
    </fieldset>
    <fieldset>
      <label for="lng" class="fixedwidth">Lng:</label>
     <input type="text" name="lng" id="lng"class="inputtext" /><br/>
    </fieldset>
    <fieldset>
      <label for="type" class="fixedwidth">Type:</label>
     <input type="text" id="type"class="inputtext" /><br/>
    </fieldset>
    <button id="FormSubmit">Add Marker</button>
</div>

<script type="text/javascript">

var latitude = document.getElementById(lat).value;
var longitude = document.getElementById(lng).value;

var reg = new RegExp("^-?([1-8]?[1-9]|[1-9]0).{1}d{1,6}");

if( reg.exec(latitude) ) {
 //do nothing
} else {
 //error
}

if( reg.exec(longitude) ) {
 //do nothing
} else {
 //error
}

</script>

^-?([1-8]?[1-9]|[1-9]0).{1}d{1,6}

if (typeof num === 'number' && num <= 90 && num >= -90)
    //valid
else
    //invalid