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HTML

              
                <form id="contact" action="./" method="post">

  <div>
    <label for="username">username</label>
    <input type="text" autocomplete="username" id="username" name="username" required />
    <p class="help">Please enter your username.</p>
  </div>

  <div>
    <label for="email">email</label>
    <input type="email" autocomplete="email" id="email" name="email" />
    <p class="help">Please enter your email or telephone.</p>
  </div>

  <div>
    <label for="tel">telephone</label>
    <input type="tel" autocomplete="tel" id="tel" name="tel" />
    <p class="help">Please enter your email or telephone.</p>
  </div>

  <div>
    <button type="submit">submit</button>
  </div>
  
</form>
              
            
!

CSS

              
                * {
  font-family: sans-serif;
  font-size: 1em;
}

div {
  width: 100%;
  max-width: 15em;
  margin: 1em auto;
}

label, input {
  display: block;
  width: 100%;
}

input {
  padding: 0.5em;
  border: 1px solid #666;
  border-radius: 4px;
}

button {
  display: block;
  margin: 0 auto;
}

.help {
  display: none;
  font-size: 0.8em;
  text-align: center;
  margin: 0.2em 0 1em 0;
  color: #c00;
}

.invalid .help {
  display: block;
}

.invalid label, .invalid input {
  color: #c00;
  border-color: #c00;
}
              
            
!

JS

              
                // custom form validation
class FormValidate {
  
  constructor(form, field) {
    
    // active form
    this.form = form;
    this.form.noValidate = true;

    // custom validation functions
    this.custom = [];

    // validate fields on focus change?
    this.validate = !!field;
    
    // field focusout event
    this.form.addEventListener('focusout', e => this.changeHandler(e) );

    // form submit event
    this.form.addEventListener('submit', e => this.submitHandler(e) );
    
  }
  
  
  // add a custom validation function
  // it's passed the field and must return true (valid) or false (invalid)
  addCustom(field, vfunc) {

    // get index
    let c = field.CustomValidator;
    if (typeof c === 'undefined') {
      c = this.custom.length;
      field.CustomValidator = c;
    }

    // store function reference
    this.custom[c] = (this.custom[c] || []).concat(vfunc);

  }
  
  
  // validate a field when focus changes
  changeHandler(e) {

    const t = e.target;
    if (this.validate && t && t.checkValidity) this.validateField(t);

  }
  
  
  // validate all fields on submit
  submitHandler(e) {

    // validate all fields
    let first, invCount = 0;
    Array.from(this.form.elements).forEach(f => {

      if (!this.validateField(f)) {

        // find first visible invalid
        if (f.offsetHeight) first = first || (f.focus && f);
        invCount++;

      }

    });

    // at least one field is invalid
    if (invCount) {

      // stop submission
      e.stopImmediatePropagation();
      e.preventDefault();

      // enable field focusout validation
      this.validate = true;

      // focus first invalid field
      if (first) {
        first.parentElement.scrollIntoView(true);
        setTimeout(() => first.focus(), 800);
      }

    }
    // form is valid - submit
    else if (this.submit) this.submit(e);

  }
  
  
  // validate a field
  validateField(field) {

    const
      parent = field.parentElement,
      c = field.CustomValidator,
      inv = 'invalid';

    field.setCustomValidity('');

    // default validation
    let valid = field.checkValidity();

    // custom validation
    if (valid && typeof c !== 'undefined') {
      valid = !this.custom[c].some(fn => !fn(field));
    }

    if (valid) {

      // field is valid
      parent.classList.remove(inv);
      return true;

    }
    else {

      // field is not valid
      field.setCustomValidity(inv);
      parent.classList.add(inv);
      return false;

    }

  }
  
}

// ___________________________________________________________________
// validate contact form
const contactForm = new FormValidate(document.getElementById('contact'), false);

// custom validation - email and/or telephone
const
  email = document.getElementById('email'),
  tel = document.getElementById('tel');

contactForm.addCustom(email, f => f.value || tel.value);
contactForm.addCustom(tel, f => f.value || email.value);

// custom submit
contactForm.submit = e => {

  e.preventDefault();

  alert('Form is valid!\n(open the console)');

  const fd = new FormData(e.target);
  for (const [name, value] of fd.entries()) {
    console.log(name + ': ' + value);
  }

}
              
            
!
999px

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