JSFiddle - React, Tailwind, and code Playground

HTML

<form method="post" action="php/cadastro.php">
    <span class="form_title">Cadastre-se</span>
    <fieldset>
        <input type="text" name="nome" class="txt_input first_input" placeholder="Nome">
        <input type="email" name="email" class="txt_input" id="email" placeholder="E-mail">
        <input type="email" name="confirmaEmail" class="txt_input" id="confirma-email" placeholder="Confirmar e-mail">
        <input type="password" name="senha" class="txt_input" id="senha" placeholder="Senha">
        <input type="password" name="confirmaSenha" class="txt_input" id="confirma-senha" placeholder="Confirmar senha">
    </fieldset>
    <fieldset>
        <input type="checkbox" name="termos">
        <p>Eu concordo com os <span>termos de uso e polĂ­tica de privacidade</span> do WEB TRAINING</p>
    </fieldset>
    <fieldset class="buttons">
        <input type="submit" name="Cadastrar" value="Cadastrar" class="log_btn" id="cadastro-btn" onclick="valida(event)">
        <input type="button" name="Voltar" value="Voltar" class="log_btn volta_btn" id="volta-btn" onclick="voltar()">
    </fieldset>
</form>

JavaScript

var email = document.getElementById('email');
var confirmaEmail = document.getElementById('confirma-email');
var senha = document.getElementById('senha');
var confirmaSenha = document.getElementById('confirma-senha');
var cadastrar = document.getElementById('cadastro-btn');

function valida() {
    if (email.value != confirmaEmail.value || senha.value != confirmaSenha.value) {
        cadastrar.preventDefault();
    }
}

function valida(ev) {
    if (email != confirmaEmail && senha != confirmaSenha) {
        ev.preventDefault();
    }
}