How to write the HTML so the JS can validate it

<span id="field_nameErr" class="error">Error Message</span> // Requierd
	<label for="field_name">Label</label> // Optional
	<input type="text" id="field_name" name="field_name"/> // Required

The validation happens by the ID in the INPUT tag. Also the validation will happen if only the element has an error message with an ID equals to ID+'Err'.