You can check if a form passes HTML validations with javascript using the checkValidity function:
<form name="login">
<fieldset>
<legend>Email</legend>
<input type="email" name="email" required />
</fieldset>
<fieldset>
<legend>Password</legend>
<input type="password" name="password" required />
</fieldset>
<div style="margin-top: 1rem">
<input type="submit" value="login" />
<button type="button" id="validate">
validate
</button>
</div>
</form>
<script>
const button = document.querySelector("#validate")
if (button) {
button.addEventListener("click", function() {
alert(document.forms.login.checkValidity() ? "Form is valid" : "INVALID");
});
}
</script>