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Because client side validation does not talk to the server, the theme (xhtml theme or css_xhtml theme) is responsible for properly manipulating the HTML DOM to display the error message inline. The JavaScript that is responsible for doing this logic is validation.js and can be found in each theme.
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If you wish to add additional validator support beyond those listed, you may override the xhtml theme teamplte form-close-validate.ftl. This file contains the JavaScript that tries to validate each user-entered value from within the browswer. The css_xhtml theme extends the xhtml theme and therefore doesn't have it's own form-close-validate.ftl template.