For static text use `data-validation-text` attribute:

    <input name="test" class="form-validate validate-test" data-validation-text="Value is invalid">
In case you're using the Form API, use `validationtext` attribute in the field XML:

    <field name="test" type="text" class="form-validate validate-test" validationtext="Value is invalid" />
If you need dynamic text based on value, there is no built-in support for this; validation handlers return only `boolean` value. You'd have to handle this yourself. One way is to change the `data-validation-text` attribute in the handler:

	document.formvalidator.setHandler('test', function(value, element) {
		if (value.length < 5) {
			element.dataset.validationText = 'Username must be at least 5 characters';

			return false;
		}

		if (value.length > 20) {
			element.dataset.validationText = 'Username must be no more than 20 characters';

			return false;
		}

		return true;
	});